http: avoid corking on "Content-Length: 0" response

http: avoid corking on "Content-Length: 0" response

We must use a normal write instead of send(.., MSG_MORE)
when writing responses of "Content-Length: 0" to avoid
the corking effect MSG_MORE provides.  We only want to
cork headers if we will send a non-empty body.

Fixes: c3eeaf664cf0 ("http: clarify intent for persistence")
This needs a proper test.