Avoid Dropping Upstream Items in mergeSource

[julmb]

From what I understand, mergeSource will await an item from upstream, and if successful, await an item from src0. If there is no item in src0, the conduit terminates. This means that the item that was already taken from upstream is now lost (note the missing item 4 in the output):

>>> let num = C.yieldMany [1 .. 10]
>>> let chr = C.yieldMany ['a' .. 'c']
>>> runConduitPure $ num .| liftA2 (,) (mergeSource chr .| C.sinkList) C.sinkList
([('a',1),('b',2),('c',3)],[5,6,7,8,9,10])

This PR adds the already consumed item back using leftover, leading to the expected result:

([('a',1),('b',2),('c',3)],[4,5,6,7,8,9,10])

Disclaimer: I am pretty new to Conduit, so it is possible that I am either misunderstanding something or that there is a good reason for the current behavior of mergeSource, in which case, feel free to correct me.

[snoyberg]

Seems like a good change. Can you add a comment to the changelog and do a minor version bump? An added test case would be great too, but not a blocker.

[julmb]

I added both a changelog entry and a test case.

[snoyberg]

Thanks!

[snoyberg]

There's a build error in the tests due to missing the liftA2 import, can you address that?

[julmb]

I pushed right as you wrote that, let's hope it works now. ;)

[snoyberg]

LGTM, thanks!