Restrict _.union and _.difference to accept arrays only
Merge branch 'master' of https://github.com/jashkenas/underscore
Clarify _.each return
Merge pull request #1471 from ryanve/master

Clarify _.each return
Tweak a bit, remove failing test.
Merge pull request #1474 from braddunbar/array-methods

Union and Difference Accept Only Arrays