Add option to disable close by dtls

Close peerconnection on DTLS.CloseNotify
could break ice restart with dtls restart,
when the dtls finger-print changed, the
browser could teardown the old dtlstransport
and establish new one then pion could close
the peerconnection and restart failed. So
browser don't do this and spec also don't
say peerconnection should close when dtls
is closed.

settingengine_test.go

@@ -394,3 +394,26 @@ func TestSetFireOnTrackBeforeFirstRTP(t *testing.T) {
 
 	closePairNow(t, offerer, answerer)
 }
+
+func TestDisableCloseByDTLS(t *testing.T) {
+	lim := test.TimeOut(time.Second * 30)
+	defer lim.Stop()
+
+	report := test.CheckRoutines(t)
+	defer report()
+
+	s := SettingEngine{}
+	s.DisableCloseByDTLS(true)
+
+	offer, answer, err := NewAPI(WithSettingEngine(s)).newPair(Configuration{})
+	assert.NoError(t, err)
+
+	assert.NoError(t, signalPair(offer, answer))
+
+	untilConnectionState(PeerConnectionStateConnected, offer, answer).Wait()
+	assert.NoError(t, answer.Close())
+
+	time.Sleep(time.Second)
+	assert.True(t, offer.ConnectionState() == PeerConnectionStateConnected)
+	assert.NoError(t, offer.Close())
+}