code: func (h *RequestHeader) SetContentLength(contentLength int) {
  	h.parseRawHeaders()
  	h.contentLength = contentLength
  	if contentLength >= 0 {
  		h.contentLengthBytes = AppendUint(h.contentLengthBytes[:0], contentLength)
  		h.h = delAllArgsBytes(h.h, strTransferEncoding)
  	} else {
  		h.contentLengthBytes = h.contentLengthBytes[:0]
  		h.h = setArgBytes(h.h, strTransferEncoding, strChunked, argsHasValue)
  	}
  }
docs: // SetContentLength sets Content-Length header value.
  //
  // Negative content-length sets 'Transfer-Encoding: chunked' header.