Add content type header for getAllQueries endpoint

[rdwebdesign]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)
• I have Signed Off all commits. (git commit --signoff)

---

What does this PR aim to accomplish?:

Fix #2178

How does this PR accomplish the above?:

Adding the correct content type to the headers.

What documentation changes (if any) are needed to support this PR?:
none.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-16-web-v5-13-and-core-v5-11-1-released/56384/1