libmediasoupclient don't support opus stereo

[buptlsp]

I use ffmpeg to push the stream and I set sprop-stereo=1. If I use mediasoup-client(js code), I can hear stereo sound. but If I use libmediasoupclient, I only hear mono sound.

I refer to the code, the problem is in src/sdp/Utils.cpp ：

sprop-stereo = 1, it's not a boolean value, but a number. so the answer sdp is not as expected.

[jmillan]

Mind you create a PR @buptlsp?

[buptlsp]

I'm busy recently. so , it may take some time for me to create pr.