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.
Merge pull request #239 from miyagawa/explicit_strategies

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