Fix more_like_this re: fields

ElasticSearch wants "mlt_fields", but more_like_this was sending "fields".
This fixes that and adds a test.

I tested it with ElasticSearch 0.17.9 and 0.19.11.
Nix the 'mlt_fields' from es_kwargs

This field doesn't need to be in both the es_kwargs and the method
arguments so nix it from es_kwargs.
Note the more-like-this arg name fix in the changelog.
Fix more_like_this "mlt_fields" param name so it actually works with ES.