Add delaySubscription(Observable<U>) overload?

[JakeWharton]

Usage of delaySubscription has started popping up in our code base, and I've noticed that we always have to define a Func0 simply to yield an observable that's either cold (in which case the function provides no value) or hot (in which case the function also provides no value). It's not clear to me why the function indirection was added in the first place as it's provided no data and Observable.defer could have served in its place.

I would like to propose adding an Observable-taking overload and replacing the Func0 variant to instead wrap its given argument in Observable.defer before passing to OnSubscribeDelaySubscriptionWithSelector.

Being able to sneak this in before 1.0.15 / 1.1.0 would also be nice 😀

[akarnokd]

See #3447. I didn't touch the current Func0 variant to avoid introducing unexpected effects to existing code.

[JakeWharton]

👍 Thanks @akarnokd!

[akarnokd]

Fix merged.