[BUG] ShowFullTextInRSS creates invalid RSS feed

[schroedingersZombie]

Describe the bug
Commit #00488d0 introduces an error, when site.params.showFullTextInRSS is st to true.

It removes the xmlns:content="http://purl.org/rss/1.0/modules/content/" attribute in the rss tag. This is required for the content:encoded tag containing the full text.

• Hugo Version [ >=0.112.4 expected]: 121.2
• Theme Version [e.g. v4.0, master, or commit-id ]: master

Steps to reproduce the behavior:
insert

params:
  showFullTextInRss: true

into hugo.yaml.
Paste https://yoursite.net/index.xml into https://validator.w3.org/feed/check.cgi

Expected behavior:
RSS feed validates successfully.

Edit: just noticed, I forgot to type a title

[Yuk-0v0]

same problem

[vindex10]

Hi! This PR fixes the problem, although it also brings a feature 🙃

#1370

[schroedingersZombie]

Thank you very much! :)