return http header with release name(s) used to construct response

[bhearsum]

This idea came up today and shortly afterwards I was helping debug why nothing was being returned for a particular update URL. It turned out that a different rule was being matched than we thought. Had we had the release name somewhere in the response (like a header!), this would've been much more obvious.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1425594)