Stop Resource defaults from sticking between requests.

Since hash values are refrences, a class constant hash containing
other hashes as values will pass them around as references. So if
constraints are set on one request, and not explicitly cleared on
the next request, they will persist until they are cleared.

Returning a hash literal from a class method is one way to generate
fresh hashes. There may be a better way.
Merge branch 'fix-default-attribute-bug-in-fauna-resource' into working