Collapsed forwarding + bugfix

This should allow for testing collapsed forwarding, and the second test makes sure the expected occurs with collapsed forwarding off.

Noticed that the collapsed forwarding option puts a version requirement on redis for 2.6, and if you attempt to turn it on with 2.4 you simply see a debug log of each state (stopping at "requesting_collapse_lock") and receive no content - I guess it stops executing entirely?

I added a quick check in requesting_collapse_lock to make sure it events "err_obtaining_collapse_lock" if there's an error trying to run redis:eval - eventually causing a 500 and the error generated by the self:e(). 

Don't know if this is the ideal way to handle it but it's the same way that failing to acquire the collapsed lock works so rolling with that for the moment ;)