rss: bot does not work for some feeds

Describe the bug For some rss feed the rss-bot does not work. It does simple send no message. But other feeds does work.

To Reproduce

create new unencrypted room.
add the rss bot integration.
subscribe https://warnung.bund.de/api31/mowas/rss/053340000000.rss or https://maintenance.rz.rwth-aachen.de/ticket/status/messages/feed.rss
wait for an update of the rss feed

Expected behavior A message if the rss feed is updated.

Additional context I was only able to reproduce this with rss feed in german language. May it happen, because the rss feed does include chars like äüöß?

matrix-org / go-neb

rss: bot does not work for some feeds #352