This commit adds the Accept-Encoding header to getPodcastFeed() with
gzip, compress, and deflate support. This allows servers to send a
compressed response that'll be decompressed by axios transparently.
Audiobookshelf is currently using axios v0.27.2, which enables the
decompress option by default. The decompress feature supports gzip,
compress, and deflate algorithms (see axios/lib/adapters/http.js).
axios v0.27.2 does not add the Accept-Encoding header to requests
automatically, so that's the responsibility of the caller.
3f7af8acfb