Add a Vary: Origin header when the response headers vary according to the origin.

[JulienPalard]

What do these changes do?

According to the fetch spec, I added a Vary: Origin when the response varies according to the origin, to avoid cache issues (like an Access-Control-Allow-Origin for a domain being cached then delivered to other origins).

Are there changes in behavior for the user?

It should fix some cache issues, typically when the responses get through an HTTP cache like Varnish.

Related issue number

Closes #351

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.