When the initial request isn't possible to send in its entirety, the
remainder of request would be delivered to the debug callback as data
and would wrongly be counted internally as body-bytes sent.
Extended test 1295 to verify.
Closes#6328
test1294: "split request" being when the entire request isn't sent in
the first go, and the remainder is sent in the PERFORM state. A GET
request is otherwise not sending anything during PERFORM.
test1295: same kind of split but with POST
Closes#5197