WebSocket CORS bug

[buu700]

Using the socket API from a file:// origin causes the following error:

Failed to load https://insight.bitpay.com/socket.io/?EIO=3&transport=polling&t=M9uM-gD: The 'Access-Control-Allow-Origin' header has a value 'null' that is not equal to the supplied origin. Origin 'null' is therefore not allowed access.

Other APIs return Access-Control-Allow-Origin: * and work as expected, but the socket returns Access-Control-Allow-Origin: null and breaks.