Fix edge case with multiple self links.
Prevent setting _links and _embedded.
Prevent deletion of _links and _embedded.
Merge commit 'refs/remotes/github/master'
Raise a KeyError when deleting reserved attribute.

Document acts as if _links and _embedded are not attributes. If we
delete another attribute that does not exist, a KeyError is raised. By
symmetry, deleting a reserved attribute should also raise a KeyError.
Extract constant for reserved attributes names.