maddec: Ensure that the audio blocks are of type bytes()

The audio blocks should be bytes() to be consistent with the other decoders.

This is required for the pyacoustid and chromaprint libraries, and fixes the following backtrace in Beets when using the 'chroma' plugin:

Traceback (most recent call last):
  File "/usr/bin/beet", line 11, in <module>
    load_entry_point('beets==1.4.6', 'console_scripts', 'beet')()
  File "/usr/lib/python3.6/site-packages/beets/ui/__init__.py", line 1256, in main
    _raw_main(args)
  File "/usr/lib/python3.6/site-packages/beets/ui/__init__.py", line 1243, in _raw_main
    subcommand.func(lib, suboptions, subargs)
  File "/usr/lib/python3.6/site-packages/beets/ui/commands.py", line 937, in import_func
    import_files(lib, paths, query)
  File "/usr/lib/python3.6/site-packages/beets/ui/commands.py", line 914, in import_files
    session.run()
  File "/usr/lib/python3.6/site-packages/beets/importer.py", line 327, in run
    pl.run_parallel(QUEUE_SIZE)
  File "/usr/lib/python3.6/site-packages/beets/util/pipeline.py", line 445, in run_parallel
    six.reraise(exc_info[0], exc_info[1], exc_info[2])
  File "/usr/lib/python3.6/site-packages/six.py", line 693, in reraise
    raise value
  File "/usr/lib/python3.6/site-packages/beets/util/pipeline.py", line 312, in run
    out = self.coro.send(msg)
  File "/usr/lib/python3.6/site-packages/beets/util/pipeline.py", line 194, in coro
    func(*(args + (task,)))
  File "/usr/lib/python3.6/site-packages/beets/importer.py", line 1339, in lookup_candidates
    plugins.send('import_task_start', session=session, task=task)
  File "/usr/lib/python3.6/site-packages/beets/plugins.py", line 452, in send
    result = handler(**arguments)
  File "/usr/lib/python3.6/site-packages/beets/plugins.py", line 124, in wrapper
    return func(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/beetsplug/chroma.py", line 143, in fingerprint_task
    return fingerprint_task(self._log, task, session)
  File "/usr/lib/python3.6/site-packages/beetsplug/chroma.py", line 212, in fingerprint_task
    acoustid_match(log, item.path)
  File "/usr/lib/python3.6/site-packages/beetsplug/chroma.py", line 65, in acoustid_match
    duration, fp = acoustid.fingerprint_file(util.syspath(path))
  File "/usr/lib/python3.6/site-packages/acoustid.py", line 321, in fingerprint_file
    return _fingerprint_file_audioread(path, maxlength)
  File "/usr/lib/python3.6/site-packages/acoustid.py", line 264, in _fingerprint_file_audioread
    fp = fingerprint(f.samplerate, f.channels, iter(f), maxlength)
  File "/usr/lib/python3.6/site-packages/acoustid.py", line 206, in fingerprint
    fper.feed(block)
  File "/usr/lib/python3.6/site-packages/chromaprint.py", line 126, in feed
    self._ctx, data, len(data) // 2
ctypes.ArgumentError: argument 2: <class 'TypeError'>: wrong type

beetbox / audioread

maddec: Ensure that the audio blocks are of type bytes() #77