rework zip fix for #537

[MainRo]

The original fix for #525 breaks sequences where some observables emit
item faster than other ones. By completing too soon, the remaining
observables cannot catchup later. A very simple case is in #578 where
the two observables to zip emit their items sequentially.

We can fix both issues by completing whenever an observable completes
and there is no queued item. Otherwise we let the remaining observables
a chance to emit new items before completion.

[MainRo]

@MichaelSchneeberger can you review this?

[coveralls]

Coverage increased (+0.01%) to 93.514% when pulling 2261a11 on MainRo:bugfix/zip into a4e84d8 on ReactiveX:master.

[dbrattli]

Looks good!

[MichaelSchneeberger]

I would have done it like this

def next(i):
    if all([len(q) for q in queues]):
        try:
            queued_values = [x.pop(0) for x in queues]
            res = tuple(queued_values)
        except Exception as ex:  # pylint: disable=broad-except
            observer.on_error(ex)
            return
  
        observer.on_next(res)
  
        # after sending the zipped values, complete the observer if at least one upstream observable
        # is completed and its queue has length zero 
        if any((done for queue, done in zip(queues, is_completed) if len(queue)==0)):
            observer.on_completed()

def completed(i):
    is_completed[i] = True

    if len(queues[i]) == 0:
        observer.on_completed()

[MainRo]

Thank @MichaelSchneeberger, your proposal is simpler and more readable. I updated the PR with it.