V2 get_transcript is asymmetrical

[aselder]

The response type of the get_transcript call in the V2 API is asymmetrical.

If the :format arg is 'txt', get_transcript returns a string containing the text transcript. while if :format is anything else, a VoiceBase::Response object is returned.

Either both should return a VoiceBase::Response, the text transcript call shouldn't overload the get_transcript method, or get_transcript should return a wrapper object compatible with both.