add test case for _.intersection
Merge pull request #1204 from kbrainwave/master

add test case for _.intersection
Fixes #1199 -- Add an indexBy function to go with groupBy and countBy.
Merge branch 'master' of https://github.com/jashkenas/underscore
Fixes #1192 -- documentation clarification for isNaN
Fixes #1159. Document that 'negative' ranges are zero-length.