Add force_encoding to @out in lib/ci/ansi2html.rb

What does this MR do?

• Add a test reproducing !30796
• Fix with force_encoding

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Assists @godfat

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

Links

• http://stackoverflow.com/questions/17936318/why-does-to-json-escape-unicode-automatically-in-rails-4
• https://mothereff.in/utf-8

What are the relevant issue numbers?

Related #30796 (closed)