Field Collapsing: Version is not visible for inner_hits

[KarstenRauch]

Elasticsearch version: 6.1 and below

Steps to reproduce:

Search with the example
https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-collapse.html#_expand_collapse_results
but set
"inner_hits":{ "version":true, ... }
Expected:
In the result "hits" below "inner_hits" is a field "_version".
Instead the result is the same as for "version":false,.

Test-Case:
InnerHitsCollapseIT.java.txt

Solution:

Add one extra line 168 in /_core/java/org/elasticsearch/action/search/ExpandSearchPhase.java
(Method org.elasticsearch.action.search.ExpandSearchPhase.buildExpandSearchSourceBuilder(InnerHitBuilder) ):
groupSource.version(options.isVersion());

[KarstenRauch]

"version":true, does not work for inner hits in case of field collapsing, but it works for other inner hit results like nested documents.