Fix failing proxy bug.

When the browser hangs up socket and we didn't manage to start responding
to request, it seems that resp parameter in 'proxyError' event is not filled out.

Sample resp: { domain: null,
  _events: { finish: [Function] },
  _maxListeners: 10,
  output: [],
  outputEncodings: [],
  writable: true,
  _last: false,
  chunkedEncoding: false,
  shouldKeepAlive: true,
  useChunkedEncodingByDefault: true,
  sendDate: true,
  _hasBody: true,
  _trailer: '',
  finished: false }

So, it doesn't have a .socket object and crashes the process:

ERROR [testacular]: [TypeError: Cannot read property 'destroyed' of undefined]
TypeError: Cannot read property 'destroyed' of undefined
    at i (/usr/lib/node_modules/testacular/lib/proxy.js:67:49)
    at EventEmitter.emit (events.js:126:20)
    at EventEmitter.emit (events.js:106:17)
    at ClientRequest.proxyError (/usr/lib/node_modules/testacular/node_modules/http-proxy/lib/node-http-proxy/http-proxy.js:186:14)
    at ClientRequest.g (events.js:192:14)
    at ClientRequest.EventEmitter.emit (events.js:96:17)
    at Socket.socketCloseListener (http.js:1378:9)
    at Socket.EventEmitter.emit (events.js:126:20)
    at Socket._destroy.destroyed (net.js:358:10)
    at process.startup.processNextTick.process._tickCallback (node.js:244:9)

So, to fix it, we use req, which is guaranteed to be filled out and .socket.destroyed is set to true in this case.