Included value of Accept header in Vary header generated for get requ…

[timothyjrogers]

…ests with differing behaviors based on Accept

Description

This pull request adds the value of the HTTP Accept header to the generated Vary header for GET requests that behave differently based on the value of Accept -- generally pages that return HTML or JSON based on the value of Accept. This is in support of external caching in front of Nginx, as described in #3512 .

I did not modify the Vary header behavior for any POST endpoints because I am not familiar enough to know which are idempotent.

• Closes Vary header should include Accept if a resource can be HTML and JSON #3512

What type of Pull Request is this?

• Bug Fix
• Enhancement
• Plumbing / Internals / Dependencies
• Refactor

Does this PR change settings or dependencies, or break something?

• This PR changes or adds default settings, configuration, or .env values
• This PR changes or adds dependencies
• This PR introduces other breaking changes

Details of breaking or configuration changes (if any of above checked)

Documentation

• New or amended documentation will be required if this PR is merged
• I have created a matching pull request in the Documentation repository
• I intend to create a matching pull request in the Documentation repository after this PR is merged

Tests

• [x ] My changes do not need new tests
• All tests I have added are passing
• I have written tests but need help to make them pass
• I have not written tests and need help to write them

[hughrun]

Thankyou!