(wip) subscribe scheduler forwarding issue

[jcafhe]

To continue the discussion started in #477, I've quickly reviewed all the operators in rx/core/operators and I suspect we may have the same issue for the following operators. ~~Also I've not checked static operators yet, so this list might not be comprehensive~~.

EDIT: already fixed operators

• concat_with_iterable: see PR #495
• join (line 108): see PR #493
• ConnectableObservable: see PR #492

delay_with_mapper (line 77)

Subscription delay observable should be subscribed with the same scheduler as the source observable. https://github.com/ReactiveX/RxPY/blob/7f6bd8614d2a6a9f109114b29b7b9910fc7b15ae/rx/core/operators/delaywithmapper.py#L77-L80

~~expand~~

~~don't know exactly, disambiguation needed~~ Seems OK https://github.com/ReactiveX/RxPY/blob/7f6bd8614d2a6a9f109114b29b7b9910fc7b15ae/rx/core/operators/expand.py#L21-L22

pairwise (line 40)

https://github.com/ReactiveX/RxPY/blob/7f6bd8614d2a6a9f109114b29b7b9910fc7b15ae/rx/core/operators/pairwise.py#L40

throttle_first (line 39)

Should be scheduler_instead ? https://github.com/ReactiveX/RxPY/blob/7f6bd8614d2a6a9f109114b29b7b9910fc7b15ae/rx/core/operators/throttlefirst.py#L39

timeout_with_mapper (line 69)

When the timeout observable completes and 'wins' the race, the other source should be subscribed with the specified scheduler. https://github.com/ReactiveX/RxPY/blob/7f6bd8614d2a6a9f109114b29b7b9910fc7b15ae/rx/core/operators/timeoutwithmapper.py#L69

to_marbles (line 57)

https://github.com/ReactiveX/RxPY/blob/7f6bd8614d2a6a9f109114b29b7b9910fc7b15ae/rx/core/operators/tomarbles.py#L57