Revive google RSS feeds

#4574 broke google RSS.

This fixes it. This is quite serious.

Fastly tests are failing; [here](https://github.com/guardian/fastly-edge-cache/blob/4c60ae686adc52a167f96c53527eec4c5a53a251/theguardiancom/src/test/scala/com/gu/NextGenIntegrationTest.scala#L60).

This PR introduced RSS on collections (Used by google news)
https://github.com/guardian/frontend/pull/2450

@gklopper @kaelig @ironsidevsquincy