Merge the Cache-Control header in PageRegular

[MarkejN]

I want to return caching information in my frontent module controller and merge it with the page's cache time, as described here: https://docs.contao.org/dev/framework/caching/#inline-fragments. For this to work, the main response from PageRegular must also have the Contao-Merge-Cache-Control header set:

contao/core-bundle/src/EventListener/SubrequestCacheSubscriber.php

Lines 72 to 74 in b706b7d

	if ($this->currentStrategy && $response->headers->has(self::MERGE_CACHE_HEADER)) {
	if ($isMainRequest) {
	$this->currentStrategy->update($response);

[Toflar]

I've already checked with @MarkejN on Slack.

Every controller must decide if it wants to give subrequests the ability to have Cache-Control headers merged. If it does not, our listener will not merge the headers so currently the entire logic is broken for regular pages.

I think this is the right place to do it at the moment. But I would also want @aschempp to review.

[leofeyer]

Thank you @MarkejN.

[aschempp]

Looks like this got broken in #3601 because the headers is only set for the FragmentTemplate now. I don't understand why that would need to be the case though.

I think this code should be added to FrontendTemplate::setCacheHeaders but unfortunately this PR was already merged after less than 24 hours 🤷