`Chunking` body only for `POST` and `PUT`.

The comment of `setContentLength` says:

-- This function is special: in a PUT or POST request, @http-streams@
-- will assume chunked transfer-encoding /unless/ you specify a content
-- length here, in which case you need to ensure your body function
-- writes precisely that many bytes.

Which kinda makes sense, except that this wasn't the case only for PUT
and POST, but for all methods apart from GET and TRACE.  However, it
doesn't make sense for HEAD/DELETE/... to have a chunked body by
default, especially considering that there is no way to change that
apart from modifying the built request.