[BUGFIX] fixes hasMany serialization for ED beta.10

Copied from:
https://github.com/jayphelps/emberfire/commit/292382e560adfe065e96722b6a690bd4860ca92f
https://github.com/firebase/emberfire/pull/122
Merge pull request #3 from RedRudeBoy/master

[BUGFIX] fixes hasMany serialization for ED beta.10