code: function playback(resHeaders, resBody) {
          if (!forceLive) {
            var headerContent = fs.readFileSync(filename + '.headers');
            resHeaders = JSON.parse(headerContent);
          }
  
          var socket = new EventEmitter();
          socket.setTimeout = socket.setEncoding = function() {};
          // Needed for node 0.8.x
          socket.destroy = socket.pause = socket.resume = function() {};
  
          req.socket = socket;
          req.emit('socket', socket);
  
          if (resHeaders.timeout) {
            socket.emit('timeout');
            req.emit('error', new Error('Timeout'));
            return;
          }
  
          if (resHeaders.error) {
            req.emit('error', resHeaders.error);
            return;
          }
  
          var res = new http.IncomingMessage(socket);
          res.headers = resHeaders.headers || {};
          res.statusCode = resHeaders.statusCode;
  
          if (callback) {
            callback(res);
          }
  
          if (!forceLive) {
            resBody = fs.readFileSync(filename);
          }
  
          req.emit('response', res);
  
          if (res.push) {
            // node 0.10.x
            res.push(resBody);
            res.push(null);
          } else {
            // node 0.8.x
            res.emit('data', resBody);
            res.emit('end');
          }
        }
docs: Only called if either the fixture with the constructed filename exists, or we're playing back passed in data.