allow replacing null keys with specified value in groupBy filter

[AlexCuse]

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.

[a8m]

@AlexCuse Thx for the PR.
Could you provide a Plunker example (plnkr.co) ?
Why not use the defaults filter for undefined values ?
e.g:

$scope.players = [
    { name: 'Gene',   team: 'alpha' },
    { name: 'George', team: 'beta'  },
    { name: 'Steve',  team: 'gamma' },
    { name: 'Paula',  team: 'beta'  },
    { name: 'Scruath',team: 'gamma' },
    { name: 'Dan'                   },
    { name: 'Alex'                  }
 ];
$scope.fallback = { team: 'others' }; 

<ul ng-repeat="(key, value) in players | defaults:fallback | groupBy: 'team'" >
  Group name: {{ key }}
  <li ng-repeat="player in value">
    player: {{ player.name }}
  </li>
</ul>

let me know what do you think...

[AlexCuse]

I didn't see the defaults filter, does seem like that would work.

Here is a plunker using defaults: http://plnkr.co/edit/rEDbqLkQ4ZaZ9z2Dlmro?p=info

And another using the null replacement syntax I added: http://plnkr.co/edit/RYc3RriigIJYgHF9vGlJ

As long as the defaults can be inlined like that (so I can tell why I am doing something special with the default value later on) I think defaults would be fine, if a bit less concise.