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
sounds good. Another question then: from the java api perspective, how good is the semantic of the `addFeatures` method? I'm referring to the fact that we consider the current array empty if it's the default set of features, otherwise we add to the existing one...Maybe we could even simplify this and just add all the time with no distinction? If one knows the default behaviour (all features) it makes more sense to use the set method to kinda clear the features and then `addFeatures`? If one adds to the default it won't have any effect though. Thoughts?