Fix hasMany not being serialized

Possible fix for the trouble experienced in issues below.

https://github.com/emberjs/data/issues/1276
https://github.com/emberjs/data/issues/1177
https://github.com/emberjs/data/issues/1269

Fixed the similar trouble I had while using DS.FixtureAdapter and loading a lot of data in ApplicationRoute's 'model' then going to a route which used that data. The parent no longer had the children when loaded since it did not save a set of child ids to the fixtures.

Ran tests (rake test) and ran 1048 tests with no failures.