util/diff not reporting when a/b prop is undefined

If both properties are undefined (both as a value and as in
!hasOwnProperty()), diff module needs to not report it.
util/diff not reporting when a/b prop is undefined

If both properties are undefined (both as a value and as in
!hasOwnProperty()), diff module needs to not report it.
Merge branch 'master' of github.com:Aintaer/nbd.js

Conflicts:
	util/diff.js
util/diff Let's make the previous fix real this time
Merge pull request #19 from Aintaer/master

util/diff bugfix