The Dynamic Client Registration spec specifies multiple response_types
and grant_types per client
(https://openid.net/specs/openid-connect-registration-1_0.html#ClientMetadata).
Since grant_types can be inferred from response_types we should be able
to support both without needing to store grant_types.
This also helps with oidc-client-js which expects a client that supports
both "id_token" and "id_token token".
We ran into a client that blindly takes the value of token_type, which
is lowercase "bearer", and passes that back in the Authorization header.
In an earlier PR #99 there seemed to be some support for this change to
simply accept "bearer" in addition to "Bearer".
