hippotat: Don't set an explicit `Content-Length'.

hippotat: Don't set an explicit `Content-Length'.

It turns out (a) that the existing Twisted machinery already sets a
`Content-Length' header on `POST' submissions, (b) that it's too stupid
to notice that the caller's already set one, and (c) that nginx rejects
`POST' requests with duplicate `Content-Length' headers with `400 Bad
Request', which turns out to be surprisingly hard to debug.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>