Fix race condition in Packet.MarshalTo()

This was already fixed by pion#168 but got lost in pion#227. in SFUs, in order
to distribute a packet to all clients, MarshalTo() is called in
parallel by multiple routines, causing a race condition because the
padding flag is dynamically set inside MarshalTo(). This is particular
annoying when running automated tests. This PR fixes the issue by
removing this write operation as discussed in pion#168.

packet.go

@@ -484,7 +484,6 @@ func (p Packet) Marshal() (buf []byte, err error) {
 
 // MarshalTo serializes the packet and writes to the buffer.
 func (p *Packet) MarshalTo(buf []byte) (n int, err error) {
-	p.Header.Padding = p.PaddingSize != 0
 	n, err = p.Header.MarshalTo(buf)
 	if err != nil {
 		return 0, err

packet_test.go

@@ -254,6 +254,7 @@ func TestBasic(t *testing.T) {
 				}},
 			},
 			Version:        2,
+			Padding:        true,
 			PayloadType:    96,
 			SequenceNumber: 27023,
 			Timestamp:      3653407706,