Support document body in the document update handler

This fix allows document update handlers to handle larger documents, which was the recommended solution by a CouchDB developer. A new parameter is added to Database.prototype.update (the body parameter), which is passed to the request function. This solves a critical problem with .update which doesn't work when the document length is larger than 8197 characters URI encoded.

Fixes the issue here:
https://github.com/flatiron/cradle/issues/270