Unexpected Response.Unmarshal behaviour

[easyas314159]

I've encountered an issue where I have an object that satisfies the json.Unmarshaler interface which is not being respected by Response.Unmarshal.

It would likely be a good idea to remove the dependency on a map[string]interface{} and then mapstructure here and instead make use of json.RawMessage. This also solves issues with decoding time.Time values as Time already implements the json.Unmarshaler interface for RFC 3339 formatted strings.