fix(session): flow frames with no handle are not errors

As indicated by the spec, a flow frame may be received without a
handle. This indicates a flow frame that should update the session
flow options only, so we shouldn't be printing out warning in that
case.

lib/session.js

@@ -333,9 +333,6 @@ Session.prototype._processFrame = function(frame) {
     if (frame.handle !== null) {
       if (this._linksByRemoteHandle[frame.handle]) {
         this._linksByRemoteHandle[frame.handle]._flowReceived(frame);
-      } else {
-        // @todo Proper error reporting.  Should we shut down session?
-        console.warn('received Flow for unknown link(' + frame.handle + '): ' + JSON.stringify(frame));
       }
     } else {
       _.values(this._senderLinks).forEach(function (senderLink) {