[#72069960] Add in cache control switch in query responses

- `fetch` Could possibly do with being refactored so that we use our cache_control class methods to serve the fetch response at all times, rather than playing around with the response headers in the method. Probably alright for the context of [this story](https://www.pivotaltracker.com/story/show/72069960) though

backdrop/read/api.py

@@ -185,15 +185,20 @@ def fetch(data_set_config):
             warning = ("Warning: This data-set is unpublished. "
                        "Data may be subject to change or be inaccurate.")
             response = jsonify(data=data, warning=warning)
+            # Do not cache unpublished data-sets
+            response.headers['Cache-Control'] = "no-cache, must-revalidate"
         else:
             response = jsonify(data=data)
+            # Set cache control based on data-set max_age
+            response.headers['Cache-Control'] = (
+                "max-age=%d, "
+                "must-revalidate" % data_set_config.max_age
+            )
 
+    # Headers
     # allow requests from any origin
     response.headers['Access-Control-Allow-Origin'] = '*'
 
-    response.headers['Cache-Control'] = "max-age=%d, must-revalidate" % \
-                                        data_set_config.max_age
-
     return response