Spellcheck does not ignore case

It seems in the course of 🐛 Spellcheck not working (part 2) Active we broke spellcheck functionality for capitalized keywords (e.g., “Development” instead of “development”). SearchStax servers expect spellcheck.q to be all-lowercase but the current code passes on the keywords unchanged as entered by the user (or, as processed by Search API processors, but having the “Ignore case” processor enabled for Solr servers is discouraged).

To fix this functionality again for setups where Spellcheck is set to be configured in SearchStax we convert the spellcheck query to lowercase.