Review media function and codec selection

[MayamaTakeshi]

I added code to dump data received by put_frame in bfsk_det.c and ws_speech_port.c.
The dump shows the data changes based on the codec.
So its seems we are responsible for doing transcoding when necessary.

But, strangely, ws_speech_server worked with both pcmu and pcma (changed codec in samples_extra/ws_speech_server.bfsk.js)
So if I am not doing transcoding, how ws_speech_server worked?