header.go
func (h *RequestHeader) SetMultipartFormBoundary(boundary string) {
	h.parseRawHeaders()

	b := h.bufKV.value[:0]
	b = append(b, strMultipartFormData...)
	b = append(b, ';', ' ')
	b = append(b, strBoundary...)
	b = append(b, '=')
	b = append(b, boundary...)
	h.bufKV.value = b

	h.SetContentTypeBytes(h.bufKV.value)
}
func (h *RequestHeader) SetMultipartFormBoundary(boundary string) {
	h.parseRawHeaders()

	b := h.bufKV.value[:0]
	b = append(b, strMultipartFormData...)
	b = append(b, ';', ' ')
	b = append(b, strBoundary...)
	b = append(b, '=')
	b = append(b, boundary...)
	h.bufKV.value = b

	h.SetContentTypeBytes(h.bufKV.value)
}
// SetMultipartFormBoundary sets the following Content-Type:
// 'multipart/form-data; boundary=...'
// where ... is substituted by the given boundary.