serve up all api json files with a valid expires http header
HTTP headers to include:
- 'Expires'
- 'Last-Modified'
The time offset for the expires should be configurable by the provider. Lets set the default to two days.
https://en.wikipedia.org/wiki/List_of_HTTP_header_fields
(from redmine: created on 2013-09-20, precedes #3906)