Move createSerializedForm to DS.Serializer.  Serializer calls the routine currently, but it's only defined in the JSON and Fixture implementations.
Fix belongsTo relationships to models with id 0
Merge pull request #876 from tricknotes/fix-belongs-to-relationships

Fix belongsTo relationships to models with id 0
Remove unused local variable
Merge pull request #877 from tricknotes/remove-unused

Remove unused local variable
Merge pull request #812 from zachriggle/master

Move createSerializedForm to DS.Serializer
Remove duplicate #createSerializedForm method from DS.Serializer
Merge pull request #880 from pangratz/remove_duplicate_createSerializedForm

Remove duplicate #createSerializedForm method from DS.Serializer
A record is found after loading its id through a hasMany relationship.

As we have now a reference for records in a hasMany relationship,
we need to make sure we load them when accessed. If the record is
`UNLOADED`, we materialize the record.

[Fixes #869]