Revoke auth method about facebook-sdk

[martey]

I think the error might be because of the additional changes you made to the error handling, which is similar but not the same as earlier commits (like bfbe593). If it was removed, the pull request might work properly.

Looking at the API documentation on the permissions connection, it looks like it is also possible to revoke specific permissions as well as deauthorizing the entire application. Could you extend your method to include this?

[seawolf42]

I can add this functionality to #264 if you'd like, but I will need a suggestion on how to test it without changing the permissions of the user ID supplied for testing (so other tests continue to pass).