Fix .replay(1) when only one value has been sent on the HotSignal

It took me a while to track this down, but it looks like `.replay(1)` on a hot signal would result in an empty `ColdSignal` if the hot signal had only ever sent one value.

/cc @jspahrsummers 
Good call, I found [and fixed](https://github.com/ReactiveCocoa/ReactiveCocoa/commit/43522eaf2968212fd18295ccf7dcd900475274b7) another bug with that test. :trollface: