code: func (ctx *ProxyCtx) Charset() string {
  	charsets := charsetFinder.FindStringSubmatch(ctx.Resp.Header.Get("Content-Type"))
  	if charsets == nil {
  		return ""
  	}
  	return charsets[1]
  }
docs: // Will try to infer the character set of the request from the headers.
  // Returns the empty string if we don't know which character set it used.
  // Currently it will look for charset=<charset> in the Content-Type header of the request.