Specification to determine under which conditions query expansion should occur.
0 - Unspecified spell correction mode. In this case, server behavior defaults
to auto.
1 - Suggestion only. Search API will try to find a spell suggestion if there is
any and put in the SearchResponse.corrected_query.
The spell suggestion will not be used as the search query.
2 - Automatic spell correction built by the Search API.
Search will be based on the corrected query if found.
