set_default on new object copies hash from existing object

I have a CouchRest Model object and use set_default to create an empty hash on the object when I create an instance. If I create an instance, I get the empty hash. Then I put an item in the hash. Then I create a brand new instance of the model object and that new instance magically has the item in its supposedly empty hash as was added to the prior object.

Code + irb session that duplicates the problem: http://pastie.org/345576

Email me if you want the source files or want me to go through an example interactively.