Change FastMethod.helper so that it can accurately look up methods which are covariant overrides.

This change eliminates a 'method not found' error that could be thrown by this helper method in the case of looking up the index of a method that is a covariant override.

It may be worth deprecating FastClass.getIndex(String, Class[]) to avoid this confusing error.
Merge pull request #5 from lukesandberg/master

Change FastMethod.helper so that it can accurately look up methods which...