Fix error when header_max is less than header_min

Problem/Motivation

When header_max is less than header_min, the following message is generated:

Warning: DOMXPath::query(): Invalid expression in Drupal\toc_api\Toc->initialize() (line 236 of …/toc_api/src/Toc.php)

Proposed resolution

When header_max is less than header_min, set header_max to equal header_min. (Having min override max matches what CSS does for min-width and max-width.)

Remaining tasks

Agree on solution. Implement.

User interface changes

None except that no error is generated in this case.

API changes

None except that no error is generated in this case.

Data model changes

None.