[Firebase/Auth] Fixed User.Reauthenticate signature

[rickclephas]

Since Firebase Authentication v6.0.0 the reauthenticate method has been broken.
This results in the following issues:

• when provided with valid credentials a cast exception is thrown
  Specified cast is not valid Exception on Reauthenticate #419
• when invalid credentials are provided the method doesn't throw the expected NSErrorException

What happend

In Firebase Auth v5.1.0 the reauthenticateWithCredential:completion: method was deprecated.

In Firebase Auth v6.0.0 the signature of the deprecated reauthenticateWithCredential:completion: method was changed to match reauthenticateAndRetrieveDataWithCredential:completion and the later was deprecated.

While updating from 5.8.1 to 6.5.0 the signature was never updated to reflect this change.

In Firebase Auth v7.0.0 the deprecated reauthenticateAndRetrieveDataWithCredential:completion method was removed.

So currently there is no working method to reauthenticate with credentials.