Allow subclasses of Sunspot::Queue::SessionProxy in without_proxy helper

To be able to customize the behaviour of the SessionProxy in your own
application, you should be able to subclass it. This changes the
without_proxy helper method to allow subclasses as well.
Merge pull request #13 from ryreitsma/master

without_proxy helper method should allow subclasses of Sunspot::Queue::SessionProxy as well