Support for sending a body or a form to document update handlers

I've added the option to pass a body or a form object through the `database.update()` helper.

If either `body` or `form` are defined in the options and are objects, they are encoded according to cradle's and request's underlying mechanisms (i.e. *application/json* for body, and *application/x-www-form-urlencoded* for form) and sent as the request body.

If they are defined but are not objects, update() behaves as usual and put them in the query string.

If body is used, form is ignored (that is, treated as a query parameter).

There are some basic test included.

(sorry for the messy commit log)