Fix error when receiving code 204

Code 204 indicates that a resource was deleted and that no body is being
returned. NetHttpPersistentResponseWrapper failed with a TypeError on
that response, since it assumed that the response would have a body.
This commit checks whether there is a body. If there isn't, it sets
@body to nil; otherwise it performs the same logic as before.
Merge pull request #10 from lmullen/204-no-body

Fix error when receiving code 204