Explicitly set content encoding

[DL6ER]

What does this implement/fix?

Fix deletion of addresses with spaces in them by avoiding unwanted encoding to application/x-www-form-urlencoded

---

Related issue or feature (if applicable): https://discourse.pi-hole.net/t/v6-bug-unable-to-delete-subscribed-lists-with-20-in-url-field/67483

Pull request in docs with documentation (if applicable): N/A

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.