more_like_this query does not work with nested data types

[fbaligand]

Elasticsearch version (bin/elasticsearch --version):
5.4.1

Description of the problem including expected versus actual behavior:
If I make a more_like_this query on a field A which is inside a "nested" B field, it returns no results.
If I remove "nested" type on B field, the more_like_this query works as expected : relevant results are returned.

=> Could you fix more_like_this query to work fine with fields which are inside a "nested" field ?

[colings86]

@martijnvg could you take a look at this?

[jpountz]

Duplicate of #21625