header.go
func (h *RequestHeader) SetUserAgentBytes(userAgent []byte) {
	h.parseRawHeaders()
	h.userAgent = append(h.userAgent[:0], userAgent...)
}
func (h *RequestHeader) SetUserAgentBytes(userAgent []byte) {
	h.parseRawHeaders()
	h.userAgent = append(h.userAgent[:0], userAgent...)
}
// SetUserAgentBytes sets User-Agent header value.