wav file header is fully inconsistent before seek fix

[ilyapashuk]

wav.Encode function tries to write all samples and then seek to the beginning of file and set correct duration in the header.

but it is not fully usefull in cases when it is used for streaming, for example wav.Encode(os.Stdout, streamer, format)

sox tool also do something like that, but if seeking is not possible, their header remains consistent, and if we make something like this:
rec -t wav - | play -t wav -
2 copies of sox will perfectly understand each other, "unknown" will be in the duration field.

with your function header is fully inconsistent.

please patch the main Encode function or add an other function that don't try to do anything with length in the header, may be simply keep it 0, and expect io.Writer instead WriteSeeker.