AmazonMp3: Handle plain text AMZ files now sent by Amazon (bgo#661467)

The Amazon store now gives us unencrypted AMZ files in some situations.
They have a slightly different format than usual : there's no XML
declaration, and the <playlist> tag doesn't have a version attribute.

That means that the detection of unencrypted files we had for debugging
purposes doesn't work, and using XspfPlaylistFormat.MagicFormat wouldn't
work either. So we just look at the first 10 bytes.

Also add another unit test with an example of a clear text AMZ file.