Many fields in response document are now arrays; lots of "Array to string conversion" errors

Problem/Motivation

I get a whole bunch of notices and warnings when I try to make a search on Drupal 7.54, apachesolr-7.x-1.x with the apachesolr_search sub-module enabled, using Solr 5.5.4 on Jetty version 9.2.13.v20150730 on Oracle's OpenJDK; Java version 1.8.0_121, configured with the configuration in the apachesolr/solr-conf/solr-5.x folder in apachesolr-7.x-1.x, and PHP 5.6.30.

Steps to reproduce

1. Prepare a Drupal 7.54 filesystem; download the apachesolr project, version 7.x-1.x-dev (or clone the project).
2. Set up a Drupal web server running PHP 5.6.30. Ensure that PHP.ini contains:
   

   error_reporting  =  E_ALL
   display_errors = On
   display_startup_errors = On
   

3. Set up a Solr server running Solr 5.5.4 on Jetty version 9.2.13.v20150730 on Oracle's OpenJDK; Java version 1.8.0_121.
4. Create a core named "collection1" with the configuration files in the the apachesolr project's apachesolr/solr-conf/solr-5.x folder. (helpful tip: https://hub.docker.com/r/mparker17/solr-apachesolr/ is pre-configured with Solr 5.5.4 on Jetty version 9.2.13.v20150730 on Oracle's OpenJDK; Java version 1.8.0_121, and the configuration files in apachesolr-7.x-1.8, which have not been changed between 7.x-1.8 and 7.x-1.x).
   You can create a core by running the following commands from the $SOLR_HOME directory:

   bin/solr start
   bin/solr create_core -c $SOLR_CORE_NAME
   bin/solr stop
   

5. Install Drupal 7 using the minimal install profile.
6. From /admin/modules, enable the "Apache Solr framework" and "Apache Solr search" modules.
7. From /admin/structure/types, create a new content type. I called mine "Test".
8. From /admin/config/search/apachesolr/settings, configure the default server to point to the Solr server and core you set up in step 3.
9. From /node/add, create some content. I made sure mine had the text Lorem ipsum in it.
10. From /admin/config/search/apachesolr, Queue all content for reindexing, then Index all queued content.
11. From /search/site, search for a keyword in your content. I searched for "ipsum".
12. Expected behaviour: You see no errors.
    Actual behaviour: You see the following errors:

    Notice: Array to string conversion in apachesolr_search_process_response() (line 1316 of sites/all/modules/apachesolr/apachesolr_search.module).
    Notice: Array to string conversion in apachesolr_search_process_response() (line 1319 of sites/all/modules/apachesolr/apachesolr_search.module).
    Warning: strtotime() expects parameter 1 to be string, array given in apachesolr_search_process_response() (line 1330 of sites/all/modules/apachesolr/apachesolr_search.module).
    Warning: strtotime() expects parameter 1 to be string, array given in apachesolr_search_process_response() (line 1337 of sites/all/modules/apachesolr/apachesolr_search.module).
    Warning: Illegal offset type in isset or empty in apachesolr_search_process_response() (line 1362 of sites/all/modules/apachesolr/apachesolr_search.module).
    Warning: strpos() expects parameter 1 to be string, array given in url_is_external() (line 2359 of includes/common.inc).
    Warning: strpos() expects parameter 1 to be string, array given in url_is_external() (line 2364 of includes/common.inc).
    Warning: preg_match() expects parameter 2 to be string, array given in url_is_external() (line 2368 of includes/common.inc).
    Warning: strpos() expects parameter 1 to be string, array given in url() (line 2255 of includes/common.inc).
    Warning: explode() expects parameter 2 to be string, array given in url() (line 2256 of includes/common.inc).
    Warning: htmlspecialchars_decode() expects parameter 1 to be string, array given in apachesolr_search_process_response() (line 1370 of sites/all/modules/apachesolr/apachesolr_search.module).
    Warning: Illegal offset type in isset or empty in entity_get_info() (line 7890 of includes/common.inc).
    Notice: Array to string conversion in apachesolr_search_process_response() (line 1316 of sites/all/modules/apachesolr/apachesolr_search.module).
    Notice: Array to string conversion in apachesolr_search_process_response() (line 1319 of sites/all/modules/apachesolr/apachesolr_search.module).
    Warning: strtotime() expects parameter 1 to be string, array given in apachesolr_search_process_response() (line 1330 of sites/all/modules/apachesolr/apachesolr_search.module).
    Warning: strtotime() expects parameter 1 to be string, array given in apachesolr_search_process_response() (line 1337 of sites/all/modules/apachesolr/apachesolr_search.module).
    Warning: Illegal offset type in isset or empty in apachesolr_search_process_response() (line 1362 of sites/all/modules/apachesolr/apachesolr_search.module).
    Warning: strpos() expects parameter 1 to be string, array given in url_is_external() (line 2359 of includes/common.inc).
    Warning: strpos() expects parameter 1 to be string, array given in url_is_external() (line 2364 of includes/common.inc).
    Warning: preg_match() expects parameter 2 to be string, array given in url_is_external() (line 2368 of includes/common.inc).
    Warning: strpos() expects parameter 1 to be string, array given in url() (line 2255 of includes/common.inc).
    Warning: explode() expects parameter 2 to be string, array given in url() (line 2256 of includes/common.inc).
    Warning: htmlspecialchars_decode() expects parameter 1 to be string, array given in apachesolr_search_process_response() (line 1370 of sites/all/modules/apachesolr/apachesolr_search.module).
    Warning: Illegal offset type in isset or empty in entity_get_info() (line 7890 of includes/common.inc).
    

Running through apachesolr_search_process_response() in a debugger suggests that almost every property of the $doc object is an array with the raw value at array index 0; but the code mostly assumes that every property contains the raw value directly.

I'm not certain why these messages haven't shown up before: my guess is that something changed in Solr 5.5, PHP 5.6, or Java 8. Regardless, Drupal's documentation on developing modules → , Acquia's documentation, and general PHP development best practices suggest that it is best to show all errors, warnings, notices, etc. when developing locally. I do this. When this module generates lots of error messages, error messages from my own code gets lost in the noise, so I'd like to fix these notices as much as possible.

Proposed resolution

To be determined.

Remaining tasks

To be determined.

User interface changes

None.

API changes

Hopefully none.

Data model changes

None.