add missing _.intersection tests
use String instead of function(o){ return '' + o; }; thanks @megawac

https://github.com/jashkenas/underscore/commit/a315e9f4473005a8310540b16d565519a9556106#commitcomment-5590098
use each (rather than _.each) internally
Merge pull request #1526 from davidchambers/each

use each (rather than _.each) internally
Merge pull request #1512 from davidchambers/intersection

add missing _.intersection tests