mix-to-mono only produces wav files

Contrary to what the manpage says, the output file name does _not_ determine the format.

```
sox -c 2 -n stereo.wav synth 5 sin 200 sin 400
sndfile-mix-to-mono stereo.wav mono.wav
sndfile-mix-to-mono stereo.wav mono.ogg
sndfile-mix-to-mono stereo.wav mono.mp3
sndfile-mix-to-mono stereo.wav mono.au
soxi mono.*
```

They are all wavs, despite the file name extensions.
