[HttpFoundation] Prevent duplicated headers when using Early Hints

[dunglas]

Q	A
Branch?	6.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	n/a
License	MIT

Currently, headers are being duplicated in the final response if a 103 response has previously been sent. This patch fixes the issue.

Before:

< HTTP/2 103
< alt-svc: h3=":443"; ma=2592000
< cache-control: no-cache, private
< date: Fri, 02 Feb 2024 15:21:00 GMT
< link: </style.css>; rel=preload; as=style
< server: Caddy
< x-powered-by: PHP/8.3.1
< HTTP/2 200
< cache-control: no-cache, private
< cache-control: no-cache, private
< content-type: text/html; charset=UTF-8
< date: Fri, 02 Feb 2024 15:21:00 GMT
< date: Fri, 02 Feb 2024 15:21:00 GMT
< link: </style.css>; rel=preload; as=style
< link: </style.css>; rel=preload; as=style

[nicolas-grekas]

Can this be covered by a test case?

[dunglas]

@nicolas-grekas the only way I can think of to test this is to add FrankenPHP to the CI to do an integration test, or to override the built-in header() function. What would be your preferred approach?

[dunglas]

Integration test added.

[nicolas-grekas]

Thank you @dunglas.