souphttpsrc: gzip content-encoding causes application/x-gzip output

Apple recommends HLS playlist to be encoded with gzip. When Content-Encoding is set to gzip, we have "application/x-gzip", not "application/vnd.apple.mpegURL". But when Content-Type is "application/vnd.apple.mpegURL", we should have "application/vnd.apple.mpegURL" output, even if Content-Encoding is gzip.