Don't reuse set offers/answers

 * Setting frees an internal structure (didn't dig into which),
   resulting in a segfault from use after free. Instead, do a serialize
   / deserialize dance on these sdp objects.

 * In practice, reuse is unlikely because it should be happening in
   another process somewhere off in the interwebs. This was only
   affecting the test harness. However, it would be good to look at it a
   little harder and see whether it can be prevented even there.

 * Part of #24

webrtc_test.go

@@ -37,8 +37,6 @@ func TestPeerConnection(t *testing.T) {
 	SetLoggingVerbosity(0)
 
 	Convey("PeerConnection", t, func() {
-		var offer *SessionDescription
-		var answer *SessionDescription
 		config := NewConfiguration(
 			OptionIceServer("stun:stun.l.google.com:19302, stun:another"))
 		So(config, ShouldNotBeNil)
@@ -180,7 +178,7 @@ func TestPeerConnection(t *testing.T) {
 			So(bob.ConnectionState(), ShouldEqual, PeerConnectionStateNew)
 
 			Convey("Alice creates offer", func() {
-				offer, err = alice.CreateOffer()
+				offer, err := alice.CreateOffer()
 				So(offer, ShouldNotBeNil)
 				So(err, ShouldBeNil)
 				So(alice.SignalingState(), ShouldEqual, SignalingStateStable)
@@ -193,7 +191,7 @@ func TestPeerConnection(t *testing.T) {
 				So(err, ShouldNotBeNil)
 
 				// Shouldn't be able to CreateAnswer
-				answer, err = alice.CreateAnswer()
+				answer, err := alice.CreateAnswer()
 				So(answer, ShouldBeNil)
 				So(err, ShouldNotBeNil)
 
@@ -207,9 +205,10 @@ func TestPeerConnection(t *testing.T) {
 				So(err, ShouldNotBeNil)
 
 				Convey("Bob receive offer and generates answer", func() {
-					err = bob.SetRemoteDescription(offer)
+					sOffer := DeserializeSessionDescription(offer.Serialize())
+					err = bob.SetRemoteDescription(sOffer)
 					So(err, ShouldBeNil)
-					So(bob.RemoteDescription(), ShouldEqual, offer)
+					So(bob.RemoteDescription(), ShouldEqual, sOffer)
 					So(bob.SignalingState(), ShouldEqual, SignalingStateHaveRemoteOffer)
 
 					answer, err = bob.CreateAnswer()
@@ -222,9 +221,10 @@ func TestPeerConnection(t *testing.T) {
 					So(bob.SignalingState(), ShouldEqual, SignalingStateStable)
 
 					Convey("Alice receives Bob's answer", func() {
-						err = alice.SetRemoteDescription(answer)
+						sAnswer := DeserializeSessionDescription(answer.Serialize())
+						err = alice.SetRemoteDescription(sAnswer)
 						So(err, ShouldBeNil)
-						So(alice.RemoteDescription(), ShouldEqual, answer)
+						So(alice.RemoteDescription(), ShouldEqual, sAnswer)
 						So(alice.SignalingState(), ShouldEqual, SignalingStateStable)
 					})
 				})