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)
since both of endAt and startAt do the same thing with the optional args, can you pull this logic into a shared private function?