Add new method JsonGenerator.isEnabled(JsonWriteFeature)

[JooHyukKim]

blocks #1197

[cowtowncoder]

As per my note on the other PR: let's not add this -- there is a reason this method is not included in JsonGenerator.
Naming here is bit unfortunate, but basically:

1. JsonGenerator is API for generators, NOT specific to. JSON, but
2. JsonWriteFeature IS JSON-specific, and should not be referenced for general, non-JSON-specific code

Instead, access from actual JSON-backed implementations (JsonGeneratorImpl) could have this method -- but it is not necessarily even needed (existing code doesn't use this).

[JooHyukKim]

As per my note on the other PR: let's not add this -- there is a reason this method is not included in JsonGenerator. Naming here is bit unfortunate, but basically:

Reasonable. CLosed this accordingly.