[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        | 
```
You can remove whole `prepare()` method declaration as it doesn't change anything now and just call parent method.