code: public WebSocketFrame setPayload(String payload)
      {
          if (payload == null || payload.length() == 0)
          {
              return setPayload((byte[])null);
          }
  
          return setPayload(Misc.getBytesUTF8(payload));
      }
docs: Set the payload. The given string is converted to a byte array
  in UTF-8 encoding.
  
  <p>
  Note that the payload length of a <a href="http://tools.ietf.org/html/rfc6455#section-5.5"
  >control frame</a> must be 125 bytes or less.
  </p>
  
  @param payload
  The unmasked payload. {@code null} is accepted.
  An empty string is treated in the same way as
  {@code null}.
  
  @return
  {@code this} object.