After answering, with a status below 400, a request whose body the
handler left unread, the server read past the body to reuse the
connection up to the payload limit (100MB by default), with only each
read bounded by the read timeout, so a slow sender could hold a worker
for a long time. It now skips at most
CPPHTTPLIB_UNREAD_PAYLOAD_MAX_LENGTH (256KB, as Go's net/http does)
within the read timeout as a whole, and closes the connection
otherwise. An error response already closed the connection.