Fixed base64Encoding deprecation, added the actual HTTP status code to connection failure NSError

Fixed base64Encoding deprecation, added the actual HTTP status code to connection failures with key ‘HTTPResponseStatusCode’ in the NSError userInfo dictionary on status codes of >= 400, gives you the ability to see why a connection fails such as a 401 authentication failure
I think it's best to expose this in the header as `SRHTTPResponseErrorKey` and change the domain to `SRWebSocketErrorDomain` while you're here.