Fix missing NoAutoCacheControl when vary on contex hash is set from Response

[andrerom]

This moves logic to set NoAutoCacheControl so it is always executed if
vary header exists, now also when that is the case from response already.

Also adds unit test coverage for this feature.

[andrerom]

@dbu I was trying to debug why I was not getting cached pages when having session, and found this in condition where you specify vary header yourself (both with always_vary_on_context_hash enabled and not).

Hope this is the right fix, if accepted it would be good to do a release to avoid issues for people upgrading to Symfony 4.1.

[dbu]

great, thanks a lot!

[dbu]

i will tag a bugfix release now.

oh, btw: it would be great if you could add changelog entries for such pull requests. i will go and add changelog entries for the things already merged.

[andrerom]

oh, btw: it would be great if you could add changelog entries for such pull requests. i will go and add changelog entries for the things already merged.

ok, I'll do that next time ;)

Maybe add a PR template with TODO elements to make it more clear btw?