code: 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)
  }
docs: // SetMultipartFormBoundary sets the following Content-Type:
  // 'multipart/form-data; boundary=...'
  // where ... is substituted by the given boundary.