Request response_mode not respected for implicit grant

[RobertMe]

According to the spec:

When using the Implicit Flow, all response parameters are added to the fragment component of the Redirection URI, as specified in OAuth 2.0 Multiple Response Type Encoding Practices [OAuth.Responses], unless a different Response Mode was specified.

But this isn't the case in the implementation, meaning it always returns a fragment. AuthGrant uses ResponseHandler to handle the requested response mode, but this can't be used for the ImplicitGrant as it is tightly coupled to the auth code flow (using AuthenticationRequest, the code, etc).