code: public void writeBodyFromString(String bodyAsString, Charset charset) {
          message.contentEncoding(charset.name())
                  .contentType(Message.TEXT_PLAIN);
          byte[] bodyContent = bodyAsString.getBytes(charset);
          message.body(bodyContent);
      }
docs: Writes the message body from a string using the given charset
  as encoding and setting the content type to text/plain.
  
  @param bodyAsString Body to write as string
  @param charset The charset to encode the string