Do not try to load unavailable Request class for the current phase.

If you send grant_type=code for token request for example, you'll see runtime
exceptions such as:

    undefined method `pre_auth'; for #<Doorkeeper::TokensController:0x007fe04b7aa778>

Since grant_type comes as a request parameter, it's better to explicitly
limit the available request implementations rather than trying to load the
class and rescue constant lookup failures.