Allow copy's filter to return a Promise

[RyanZim]

No description provided.

[manidlou]

This is good @RyanZim! But, you mean we don't need to check if we have a filter opt at all? I mean calling Promise.resolve without checking whether we need to call it or not, wouldn't affect the perf for the cases that we don't have a filter opt?!

[RyanZim]

@manidlou Good point about the perf; wouldn't have much impact if we're just calling a noop function, but with the extra Promise.resolve layer, that will probably have a perf hit.

[RyanZim]

Updated & fixed merge conflicts.

[coveralls]

Coverage decreased (-0.04%) to 86.07% when pulling 9732252 on async-filter into 03fba97 on develop.

[manidlou]

LGTM.