Add failing spec to demonstrate issue with attribs

When a record is fetched from the server, any attributes the server
returns that are not declared on the Spine model are preserved and
accessible on the client side. Examples might include Rails' :created_at
and :updated_at fields.

However, if a record is created client-side and saved to the server,
Spine does not allow those additional undeclared attributes to get set
on the record.
Add #refresh method to Model instances to allow full updates from JSON

Resolves #486 by traversing the model instance and loading fresh data down on the root record.
Return `this` instead of `root`, per comments in #486
Bring attributes branch up to speed with dev
Merge pull request #486 from jcarlson/attributes

Better support for read-only attributes sent back by the server