Feeds doesn't respect RSS codepage

**Describe the bug**
I added a couple of RSS feeds and realized that the bot doesn't respect the encoding set in the source.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to Feeds
2. Add a new feed with URL https://www.opennet.ru/opennews/opennews_all.rss Use the template $FEEDNAME: $LINK. $SUMMARY
3. Wait till the new entry arrives
4. See error

**Expected behavior**
Correct converting feed entries to UTF-8 from the encoding set by the source feed

**Screenshots**

**Client:**
- Matrix Client and Version: Element 1.6.20
- Browser: Firefox

**Additional context**
This particular feed has the encoding set in its header: ``, but it's being ignored by the bot. Another one I tried had encoding="windows-1251", and it didn't work right either