code: func (h *RequestHeader) MultipartFormBoundary() []byte {
  	b := h.ContentType()
  	if !bytes.HasPrefix(b, strMultipartFormData) {
  		return nil
  	}
  	b = b[len(strMultipartFormData):]
  	if len(b) == 0 || b[0] != ';' {
  		return nil
  	}
  
  	var n int
  	for len(b) > 0 {
  		n++
  		for len(b) > n && b[n] == ' ' {
  			n++
  		}
  		b = b[n:]
  		if !bytes.HasPrefix(b, strBoundary) {
  			if n = bytes.IndexByte(b, ';'); n < 0 {
  				return nil
  			}
  			continue
  		}
  
  		b = b[len(strBoundary):]
  		if len(b) == 0 || b[0] != '=' {
  			return nil
  		}
  		b = b[1:]
  		if n = bytes.IndexByte(b, ';'); n >= 0 {
  			b = b[:n]
  		}
  		if len(b) > 1 && b[0] == '"' && b[len(b)-1] == '"' {
  			b = b[1 : len(b)-1]
  		}
  		return b
  	}
  	return nil
  }
docs: // MultipartFormBoundary returns boundary part
  // from 'multipart/form-data; boundary=...' Content-Type.