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 only the hot signal had only ever sent one value.

/cc @jspahrsummers 
I don't think this fix is strictly correct. `lastIndex` is meant to mean “the index of the last value sent,” so we shouldn't re-send values that appear at that index.