The payload limit counts only chunk data, and a chunk-size line is only
bounded by the line length limit, so a peer could send a few bytes of
data with any amount of padding, leading zeros and chunk extensions
around them. Count those bytes as Go does: each chunk may spend 16
bytes plus twice its data on its size line and the CRLF after it, which
still lets a stream send a byte at a time, and the excess may build up
to CPPHTTPLIB_HEADER_MAX_LENGTH before the read fails. This covers
request bodies on the server and response bodies on the client.