workaround for issues with opening data channels after data channels have closed

(workaround for https://code.google.com/p/webrtc/issues/detail?id=3778)
Bah, a nuisance to get this working right. I think the situation can be summed as follows:

> If an `RTCPeerConnection` has ever opened a data channel and any data channel has been closed (by either peer) then the *next* call to `createDataChannel` will silently fail.

So, we need to inject a dummy call to `createDataChannel`.

Tested with the SOCKS server in uproxy-networking.
yeah, good point, it might actually be good documentation.