GitHub issue #11: Nevow should handle interrupted HTTP responses.

There is nowhere in Nevow to attach an error handler for `Request.finish()` being called after a connection has been dropped, because the call is inside Nevow itself. A web application that uses redirection will get an error every time it does any asynchronous rendering, because the redirection happens before rendering finishes. This patch has Nevow catch and ignore the error.
Could you please add a docstring for this test?