Elasticsearch: Cast __in lookup values to list

This makes sure lazily-evaluated lists (like querysets) are evaluated so they can be converted to JSON
failing test for #465
Merge branch 'es-fix' of https://github.com/kaedroho/wagtail into kaedroho-es-fix

Conflicts:
	wagtail/wagtailsearch/backends/elasticsearch.py
changelog entry for #465
Merge branch 'kaedroho-es-fix'