use routing and preference of original SearchRequest in collapse innerhits requests

[jgaedicke]

We use the collapse feature with inner_hits as described here : https://www.elastic.co/guide/en/elasticsearch/reference/5.6/search-request-collapse.html

Is there a reason why the routing and preference settings of the main search request are not used for the inner hit queries. I think it would be a valid optimization to preserve these settings.

see org.elasticsearch.action.search.ExpandSearchPhase

elasticsearch/core/src/main/java/org/elasticsearch/action/search/ExpandSearchPhase.java

Line 92 in ec64c2c

SearchRequest groupRequest = new SearchRequest(searchRequest.indices())