fix bug where plugins with search_fulltext = False were rendered

Add tests for the default case and when they should be skipped (with
search_fulltext set on the plugin or the model).

Split the get_search_data() method so it is easier to test the logic
with less set up.
Merge branch 'fix_search_fulltext' of github.com:aptivate/aldryn-search into aptivate-fix_search_fulltext

Conflicts:
	aldryn_search/search_indexes.py