message: [BUGFIX] delete old peerConnection.
  ATM, in a p2p call, if you unsubscribe to a stream and you try to re-subscribe you will get an error.
diff: @@ -84,7 +84,10 @@ Erizo.Room = function (spec) {
  stream.hide();
  // Close PC stream
  - if (stream.pc) stream.pc.close();
  + if (stream.pc) {
  + stream.pc.close();
  + delete stream.pc;
  + }
  if (stream.local) {
  stream.stream.stop();
  }
  