Stop extending if `String` flag is `false`

Allow `Ember.EXTEND_PROTOTYPES.String` is `false`.
Merge pull request #1319 from tricknotes/option-for-extending-string

Stop extending if `String` flag is `false`
Fix async belongsTo to return the record when data it's already loaded
Merge pull request #1320 from andrei1089/master

Fix async belongsTo to return the record when data it's already loaded