Download buffering not working with curl src

For some reason, the souphttpsrc element got blacklisted in my setup, and I ended up spending quite some time finding why download buffering wasn't working as expected.

1. Launch GDK_BACKEND=x11 ./playback-test 0 https://trailers.apple.com/movies/wb/aquaman/aquaman-trailer-3_h480p.mov
2. Select the Buffering toggle in the playbin options
3. Press play

What should happen: Download buffering, can seek in the already downloaded portion of the video

What happens: Can't seek at all, download buffering doesn't happen

Would be nice if playbin threw an error when the HTTP source being used doesn't support download buffering, or if whatever is needed to make download buffering work was made mandatory for every HTTP source.