Allow different search types in Multisearch queries

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-request-body.html#_parameters_4
The ```search_type``` can't be in the search request body. 

The default one is ```query_then_fetch``` (http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-request-search-type.html#query-then-fetch), and this will also be the one running by default in multisearch queries