[API] Improved the `__extract_parts` utility method to handle Boolean and String values differently

    __extract_parts({ :foo => true }, [:foo])
    # => ['foo']

    __extract_parts({ :foo => 'qux'}, [:foo])
    # => ['qux']
[API] Updated the "Indices Stats" API

Related:

* elasticsearch/elasticsearch#6390
* elasticsearch/elasticsearch@bb15def