WebSockets don't work on WinRT, connection time outs

On Windows 10 and Qt 5.6.0, apps that are using websockets launch successfully, but hang in «connecting» state, and then time out into an «error» state.

That affects even trivial testcases that just try to conntect to ws://echo.websocket.org.

Everything works fine on the same setup with a desktop build (not WinRT).

This affects both old WinRT (for 8.1) and the new one (for 10), both 32 bit and 64 bit.

Network tests work (including XMLHttpRequest from qml), websockets from WebView also work.