Rxswift: Allow BehaviorSubject without initial value

[hannesstruss]

In some cases it is convenient to initialize an empty BehaviorSubject without an initial value.

[kzaher]

Hi @hannesstruss ,

then you want ReplaySubject.create ....

[hannesstruss]

Indeed. Thanks!

[menehune23]

Don't forget to set the buffer size to 1 if you want exactly a BehaviorSubject minus the initial value piece.