StreamFromSignal with onlyChanges = False may filter out initial value

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onlyChanges = False option is not used by Airstream itself, and is not directly exposed in Airstream's intended public API, but still, it was intended as an escape hatch, and should work.

Discussion in discord

See failing test case in streamfromsignal-fix branch

I think the solution is something along the lines of adding "if isFirstPull and !changesOnly, then run that same transaction" to def onStart in class StreamFromSignal, but I would need to do a more thorough review to confirm.