Only coerce belongsTo ids to string if id is defined
Fix RestAdapter#ajax
Merge pull request #938 from teddyzeenny/fix-ajax

Fix RestAdapter#ajax to account for empty hash
use store#recordWasError hook
Merge pull request #939 from tchak/became-error

use store#recordWasError hook
Merge pull request #937 from teddyzeenny/fixture-belongs-to

Only coerce belongsTo ids to string if id is defined