Fixing primary key problem for FixtureAdapter when the key normaly is generated on the server.

There where an error that could trigger if you created a record and then commited it.
You also needed to define a primaryKey field. And not set the primary key when the 
record was created. This worked fine with the RESTAdepter, since the primaryKey was 
set by the server.

"""
 assertion failed: An adapter cannot assign a new id to a record that 
 already has an id. <Models.MyModel:ember2545> had id: ember2545 and you
 tried to update it with undefined. This likely happened because your
 server returned a data hash in response to a find or update that had a
 different id than the one you sent.
"""