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 
https://github.com/github/swift-style-guide/#when-specifying-a-type-always-associate-the-colon-with-the-identifier