Crash when proxied request is hung up by browser.

When the browser hangs up socket and the proxied server didn't manage to start responding, 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 }
```
As we see, 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.