Update with normalize

I've also cherry picked [@rwjblue's PR](https://github.com/emberjs/data/pull/1791) even if I'm not sure if updatePayload is worth it.

I've done it considering [@bmac comment](https://github.com/emberjs/data/pull/2096#issuecomment-49619854)
@sly7-7 I realize this came from @rwjblue's commit but could you update this test description to say something like "Calling updatePayload allows partial updates with raw JSON"? Currently `_partial` is mentioned in the description but it isn't used directly in the test.