song/DetachedSong: copy the AudioFormat from LightSong

Enables the "Format" row in "playlistinfo" responses.

https://github.com/MusicPlayerDaemon/MPD/issues/1094

src/SongSave.cxx

@@ -86,8 +86,7 @@ song_save(BufferedOutputStream &os, const DetachedSong &song)
 
 DetachedSong
 song_load(TextFile &file, const char *uri,
-	  std::string *target_r,
-	  AudioFormat *audio_format_r)
+	  std::string *target_r)
 {
 	DetachedSong song(uri);
 
@@ -113,13 +112,11 @@ song_load(TextFile &file, const char *uri,
 			if (target_r != nullptr)
 				*target_r = value;
 		} else if (StringIsEqual(line, "Format")) {
-			if (audio_format_r != nullptr) {
-				try {
-					*audio_format_r =
-						ParseAudioFormat(value, false);
-				} catch (...) {
-					/* ignore parser errors */
-				}
+			try {
+				song.SetAudioFormat(ParseAudioFormat(value,
+								     false));
+			} catch (...) {
+				/* ignore parser errors */
 			}
 		} else if (StringIsEqual(line, "Playlist")) {
 			tag.SetHasPlaylist(StringIsEqual(value, "yes"));