make Opus SDP always return 48khz and 2 channels (#204)

RFC7587 mandates 48khz as sample rate and 2 channels inside the SDP.
These values can be dynamically adjusted by the stream, but they must
not be touched inside the SDP.

pkg/format/format_test.go

@@ -296,9 +296,8 @@ func TestNewFromMediaDescription(t *testing.T) {
 				},
 			},
 			&Opus{
-				PayloadTyp:   96,
-				SampleRate:   48000,
-				ChannelCount: 2,
+				PayloadTyp: 96,
+				IsStereo:   true,
 			},
 		},
 		{