Remove accidental global.
Merge pull request #1604 from braddunbar/accidental-global

Remove accidental global.
fix treatment of null/undefined destination in _.defaults and _.extend
respect iterator return value of Infinity/-Infinity in _.min/_.max
Merge pull request #1607 from davidchambers/extend-null

fix treatment of null/undefined destination in _.defaults and _.extend
Merge pull request #1606 from davidchambers/infinity

respect iterator return value of Infinity/-Infinity in _.min/_.max