Polish collection usage in HttpHeadersTests

[codingkiddo]

This PR performs a small test-only readability cleanup in HttpHeadersTests.

Changes:

• Replaces selected Arrays.asList(...) usages with List.of(...) where nulls and mutability are not required.
• Simplifies small test collection setup where applicable.
• Keeps production code unchanged.

No behavior change intended.

[bclozel]

@codingkiddo please run the build before submitting a pull request. You can do so with ./gradlew check locally. Once fixed, you can push additional commits to your branch to update this PR.
Also, please use a valid email when signing your commits. This doesn't need to be a public email, so an anonymized GitHub email works fine as long as GitHub associates it with your account. See https://spring.io/blog/2025/01/06/hello-dco-goodbye-cla-simplifying-contributions-to-spring/ for more