API handling of Authorization Code breaks RFC2617

The handling of the authentication realm in lib/pleroma/plugs/oauth_plug.ex is case sensitive, contrary to https://tools.ietf.org/html/rfc2617#section-1.2, which states that the realm directive (Bearer, Token, Basic &c) should be case insensitive

In auth_plug.ex the offending line 16 is:

["Bearer " <> header] -> header