Cast to string in strip_tags

[dbosen]

Make sure these boxes are checked before submitting your pull request - thank you!

• All coding styles are fulfilled. (How to check for cs issues?)
• All tests are running locally. (How to run the test?)
• Necessary update hooks are provided.
• User roles have correct access for new introduced permission.
• Every thunder module has a README.md in its root. Follow this guidelines, but we don't need every topic.
• Code is covered with well-balanced amount of inline comments.
• New features or changes are documented.

If you are really awesome, then your feature is covered by additional tests. Well done!