[HttpFoundation] Fixed messed up headers

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

I ran into this when somewhere in our code the Response object was cast into a string. After that, when send()'ing the Response, the headers would be all mixed up. This is because the __toString in HeaderBag ksorts the headers, while the __toString in ResponseHeaderBag doesn't ksort the corresponding headerNames. This patch ksorts that one as well.