code: func isTextContent(h http.Header) bool {
  	for _, header := range h["Content-Type"] {
  		for _, good := range textContentTypes {
  			if strings.HasPrefix(header, good) {
  				return true
  			}
  		}
  	}
  	return false
  }
docs: // isTextContent returns True if Content-Type header corresponds to some
  // readable text type.