Passing non-existent credentials should return null user

[AfzalivE]

In getUserByIdentity, passing some credentials that don't exist like "token" actually returned a valid user when it shouldn't.

This commit fixes this behaviour.

Also reverted the previous commit which tried to fix #215 , that behaviour was also caused by this problem and seems to be fixed now.

[vgoodvin]

+1
When I pass wrong credentials to ConfideEloquentRepository::getUserByIdentity() it returns first user from DB.
Example credentials:

array(
    'email'    => null,
    'username' => null,
);