Merge e8bebe0b03d256371c38a983d7a6d2147d827cba into 3eb8d22ddb8982ca4fb56bb7a8d6517538bf14c6

Why?
Options like `preferredquality` are ignored when postprocessing mp3s.

What?
Remove `mp3` from the list of `filecodec` values to check for in the
"lossless" condition unless `preferredquality` is unspecified.

youtube_dl/jsinterp.py

@@ -686,6 +686,8 @@ class JSInterpreter(object):
             raise self.Exception('Cannot get index {idx!r:.100}'.format(**locals()), expr=repr(obj), cause=e)
 
     def _dump(self, obj, namespace):
+        if obj is JS_Undefined:
+            return 'undefined'
         try:
             return json.dumps(obj)
         except TypeError:

youtube_dl/postprocessor/ffmpeg.py

@@ -276,7 +276,10 @@ class FFmpegExtractAudioPP(FFmpegPostProcessor):
                 acodec = 'copy'
                 extension = 'm4a'
                 more_opts = ['-bsf:a', 'aac_adtstoasc']
-            elif filecodec in ['aac', 'flac', 'mp3', 'vorbis', 'opus']:
+            elif (
+                filecodec in ['aac', 'flac', 'vorbis', 'opus'] or
+                (filecodec == 'mp3' and self._preferredquality is None)
+            ):
                 # Lossless if possible
                 acodec = 'copy'
                 extension = filecodec