Fix to _flush

[terreng]

This fixes a bug for me where the steam would never end for certain files.

When doing something like

response.body
.pipe(transcoder)
.pipe(new dfpwm.Encoder())
.pipe(fs.createWriteStream(path.join(temp_dir, 'output.dfpwm')))
.on('finish', function() {
    console.log('Done')
})

it would complete and the file would be written successfully but would never call finish.

This is fixed with a slight modification to _flush in the encoder class to push the final encoded buffer to the stream before calling the callback fn.