Added response property for scoketio to work (Fixes #501)

Socketio client looks for xhr.response property which the fake xhr doesnt have on it.

Quick catch here is: when ever you do a initiate a socketio server on client side always set the `forceBase64` to `true`.

Example:
`var socket = io("http://localhost:3000", {
        "forceBase64" : true
    });`
Merge pull request #503 from gyandeeps/master

Added response property for scoketio to work (Fixes #501)