code: function transform(req, res, next) {
      const state = res.state;
      const body = state.body;
      const type = typeof body;
      const isBuffer = body instanceof Buffer;
      let contentType;
  
      // error conversion
      if (body instanceof Error) {
          res.log('transform', 'Converting Error to response');
          res.status(500).body(httpStatus[500]).set('content-type', 'text/plain');
  
      // buffer conversion
      } else if (isBuffer) {
          res.log('transform', 'Converting Buffer to base64 string');
          res.body(body.toString('base64'));
          contentType = 'application/octet-stream';
  
      // object conversion
      } else if (type === 'object') {
          res.log('transform', 'Converting object to JSON string');
          res.body(JSON.stringify(body));
          contentType = 'application/json';
  
      // not string conversion
      } else if (type !== 'string') {
          res.log('transform', 'Converting ' + type + ' to string');
          res.body(String(body));
          contentType = 'text/plain';
  
      } else {
          contentType = 'text/html';
      }
  
      // set content type if not yet set
      if (!state.headers.hasOwnProperty('content-type') && contentType) {
          res.log('transform', 'Set content type');
          res.set('Content-Type', contentType);
      }
  
      next();
  }
docs: Response middleware for transforming the response body and setting content type.
  @param {Request} req
  @param {Response} res
  @param {function} next