CodeResponse is taking responsibilities from TokenResponse

As described in https://github.com/doorkeeper-gem/doorkeeper/issues/398,
sometimes tokens are trated as access grants. This is because at some point we
are handling a response with `Code` rather than `Token` (when requesting
http://localhost:3000/oauth/authorize?client_id=dd5c473a518f28644aa1a90705b8f4781dacdd5b2a4f4c96f8ab269aa364574d&redirect_uri=urn:ietf:wg:oauth:2.0:oob&response_type=token
for example).

This branch is a quick and dirty spike. The real solution should instantiate a
TokenResponse when `response_type` is `token`.