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