Improve #137 with explicitly nil keys

Trying to see how sporkmonger's [comment](https://github.com/sporkmonger/addressable/issues/137#issuecomment-38083363) could be addressed:

> I could go either way on that, though I think I would lean toward {"page"=>nil, "per_page"=>"20"} being the more correct return value.

I don't know if this is the best fix... I didn't spend too much time trying to grok the code.  It's a one-line change that passes all the tests, though.