Fix bug in `nextUntil`

Ensure that the `nextUntil` method operates over every element in the
collection, and avoid returning a collection with duplicate elements.
Fix bug in `prevUntil`

Ensure that the `prevUntil` method operates over every element in the
collection, and avoid returning a collection with duplicate elements.
Fix bug in `next`

Ensure that the `next` method operates over every element in the
collection.
Fix bug in `nextAll`

Ensure that the `nextAll` method operates over every element in the
collection, and avoid returning a collection with duplicate elements.
Fix bug in `prev`

Ensure that the `prev` method operates over every element in the
collection.
Fix bug in `prevAll`

Ensure that the `prevAll` method operates over every element in the
collection, and avoid returning a collection with duplicate elements.
Merge pull request #340 from jugglinmike/nexts-and-prevs

Fix bugs in "next*" and "prev*" methods