[HttpFoundation] Improve string values in test codes

I made the meanings of string literals more clear.

| Q             | A
| ------------- | ---
| Bug fix?      | no,
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a