refactor(ReplaySubject): rename an argument to ReplaySubject's constructor
refactor(ReplaySubject): rename an argument to ReplaySubject's constructor

Rename constructor argument _windowTime to windowTime, for better developer experience when using
IDEs that suggest usage by showing a dropdown autocomplete menu. This might also positively impact
documentation, which will should show windowTime instead of _windowTime. The underscore was only
needed in order to avoid collision with the operator method windowTime in Observable.

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