fix(server): properly close flash transport

Not sure if there is another way of closing socket.io flash transport properly... Above all, I think it should be properly fixed in the socket.io in the first place, but given that 0.9.x of socket.io haven't seen any updates in _months_ I'm not positive about fixing it in the socket.io.

Anyway, here it is, would be grateful for any feedback. This is part of fixes to get the https://github.com/karma-runner/karma/issues/1035 sorted out.