Patch 'endAt' and 'startAt' with no argument

the method 'endAt' and 'startAt' in firebase convert null parameter to string, because i think that it check the number of parameter passed. to patch this i have create a temporary list to pass to the method only the defined argument.

normaly, it patch the [issue#8](https://github.com/firebase/firebase-dart/issues/8)
How about "_removeNulls" ? That way it's self-documenting and then you can remove the comment.