code: function handleReply(inboundMessage, data) {
    if (data.reply) {
      _.merge(data.content.headers, Transport.getReplyHeaders(inboundMessage));
  
      data.content.subject = Transport.getReplySubject(inboundMessage);
  
      delete data.reply;
    }
  
    return data;
  }
docs: Modifies the data to be in reply if in reply
  
  @param {inboundMessage} inboundMessage
  @param {Object} data - the data for the SparkPost API
  @returns {Object} data