Check HTTP Status Code for XHR Polling Transport

Adding check for HTTP status code of NSURLConnection responses during xhr polling transport just like when checking HTTP status code during handshake (https://github.com/pkyeck/socket.IO-objc/commit/317b9e683a4eee309cf41639bf9b4ebb3b05fff3). This handles case when server restarts while connection via polling with device is already established.

When using xhr polling with an established connection and then restarting my server, the library would get in an infinite loop of receiving 502 Errors and passing the error html response data onto my app. Added check for response status code in SocketIOTransportXHR.m and throw error accordingly.

Possibly Related to Issues: https://github.com/pkyeck/socket.IO-objc/issues/87, https://github.com/pkyeck/socket.IO-objc/issues/76

Original Library Issue for 503 Error: https://github.com/pkyeck/socket.IO-objc/issues/36