Indices API: Fix GET index API always running all features

Previous to this change all features (_alias,_mapping,_settings,_warmer) are run regardless of which features are actually requested. This change fixes the request object to resolve this bug
I'm not a big fan of this change, especially because of the differences between request and request builder. Shall we try and move the addFeatures down to the request? Also, would it make sense to change the features array to become a list and use an enum (and maybe have a special value for all which could be the default value)?