allow replacing null keys with specified value in groupBy filter

I gave up on trying to figure out why, but the combination of groupBy, toArray and angular's built-in orderBy filter had some disastrous side effects for me.  So added an option to replace null/undefined keys as a means to control where missing keys fall in the natural ordering.

It seems to build OK and the tests pass, I just didn't check the combined files in because a "reverseArray" function that I didn't touch is showing as removed in angular-filter.js.