Improve assertions for _.compact()
Improve assertions for _.compact()

Explicitly test all falsy values, and assert it works with map.

documentcloud/underscore,documentcloud/underscore,vincent1988/underscore,vincent1988/underscore,tnga/underscore-es,akre54/underscore,platinumazure/underscore,platinumazure/underscore,tnga/underscore-es,jdelbello/underscore,keepgoingwm/underscore,akre54/underscore,huangnan1/2016gitStudy,jashkenas/underscore,keepgoingwm/underscore,rlugojr/underscore,jpalomar/underscore,jpalomar/underscore,rlugojr/underscore,jashkenas/underscore,jdelbello/underscore,huangnan1/2016gitStudy