Don't parse Rinku returned value to DocFragment when didn't change original html string.

What does this MR do?

Avoid parsing html string to doc fragment when Rinku.auto_link does not modify the html string

Why was this MR needed?

To try to reduce time on AutolinkFilter calls

What are the relevant issue numbers?

#18593 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together