Adds tests for user extra attribute behavior

Extra attributes (not directly defined on the model) are stashed away
in a JSON blob.  When returning a user entity they are pulled from the
blob and added directly to the entity.

Change-Id: Ic85b692dda2530efa2ef9833a8a8eed0b12bc7fb
Related-Bug: #1214686
Merge "Adds tests for user extra attribute behavior"