Don't expect 'identity' function for mapLimited() usage test
Correctly test Collection#filter/filterOut
Fix Collection#filter/filterOut

Because these methods rely on mapLimited() we need to map to the original item if it's intended to be in the filtered output; we can use a special marker object for when to skip an item.

Removes unused helper methods.
Test Collection#filter/filterOut with the iterator returning a promise
Merge pull request #6 from novemberborn/collection-filtering

Fix collection filtering