support for http caching the product catalog

[jmcallister]

We would like to cache the rest api product catalog response. The response headers do not support http caching

GET /rest/v1/catalog/products?pageSize=10 HTTP/1.1

REQUEST HEADERS
Host: apisandbox-api.zuora.com
Accept: application/json
Content-Type: application/json; charset=utf-8
Cache-Control: no-cache

RESPONSE HEADERS
Cache-Control → max-age=0, no-cache, no-store
Connection → keep-alive, Transfer-Encoding
Content-Type → application/json;charset=utf-8
Date → Thu, 21 Apr 2016 18:01:53 GMT
Expires → Thu, 21 Apr 2016 18:01:53 GMT
Pragma → no-cache
Server → Zuora App
Transfer-Encoding → chunked

It would be helpful to return a document etag and support the If-None-Match header in subsequent requests, or support the If-Modified-Since header.  Wth either of these the API can return a 304 Not Modifed response if the catalog has not changed.