Allow `recordIsLoaded` to be called with a string for the type.

`recordIsLoaded` did not support execution with a string as the `type`
argument, only a DS.Model class reference. This commit makes either
form possible. Consistent with `find`, the `type` argument is
passed through `modelFor` to resolve a class reference.
Merge pull request #1536 from lukemelia/record-is-loaded-with-string

Allow `recordIsLoaded` to be called with a string for the type.
remove unneeded closure.
Ember.computed's first property can also be the DK
Fix property order in code comment

The property order should be `type` -> `hash` -> `prop`.
Merge pull request #1538 from tricknotes/fix-property-order-in-code-comment

Fix property order in code comment
[BUG] Fix Serialization of Async belongsTo

belongsTo associations are promises, and their constructor were not being
identified correctly when piecing together its type.

This change fixes this issue in both the REST Serializer, and AMS
Serializer first by ensuring the belongsTo association is resolved. I've
also refactored the two serializers to share more common knowledge about
this activity.
WIP