Add test showing case where -bind: doesn't stop
Test that stopping -bind: disposes source signal
Remove original coarser demonstration test
Dispose of source signal when -bind: is stopped
Test -bind: stopping more thoroughly
Where did these spaces come from?
Same tests, reorganized for slightly better coverage

Now each stopping test (nil signal, stop = YES) will check that live
signals sent prior to stopping the -bind: still continue to send values
through the -bind: signal.
s/assumed/used
Merge pull request #865 from ReactiveCocoa/bind-stopping

Behavior of -bind: stopping