[HttpFoundation] Allow Cache-Control headers on StreamedResponse
feature #12198 [HttpFoundation] Allow Cache-Control headers on StreamedResponse (dhotson)

This PR was squashed before being merged into the 2.6-dev branch (closes #12198).

Discussion
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[HttpFoundation] Allow Cache-Control headers on StreamedResponse

StreamedResponse currently always sets `Cache-Control: no-cache` headers, which prevents all caching of streamed responses.

This change removes this limitation to allow normal cache control response behaviour.

Some caching proxies support caching streamed responses using chunked encoding, most notably AWS CloudFront:
http://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/RequestAndResponseBehaviorCustomOrigin.html#ResponseCustomTransferEncoding

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/6530
| License       | MIT
| Doc PR        |

Commits
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dd7a9b6 [HttpFoundation] Allow Cache-Control headers on StreamedResponse