Ensure that Rack::ETag doesn't buffer responses

In rack/rack#1416 the Rack::ETag middleware changed it's behaviour
to buffer the response and calculate an ETag digest even though
the Cache-Control: no-cache header is set. This breaks the CSV
streaming responses so to avoid that we use the hack outlined in
rack/rack#1619 and set a Last-Modified header which triggers the
skip_caching? condition in the Rack::ETag middleware.

app/controllers/admin/archived/petitions_controller.rb

@@ -112,9 +112,9 @@ def set_file_headers
   end
 
   def set_streaming_headers
-    #nginx doc: Setting this to "no" will allow unbuffered responses suitable for Comet and HTTP streaming applications
-    headers['X-Accel-Buffering'] = 'no'
+    headers["X-Accel-Buffering"] = "no"
     headers["Cache-Control"] ||= "no-cache"
+    headers["Last-Modified"] = Time.current.httpdate
     headers.delete("Content-Length")
   end
 

app/controllers/admin/petitions_controller.rb

@@ -95,9 +95,9 @@ def set_file_headers
   end
 
   def set_streaming_headers
-    #nginx doc: Setting this to "no" will allow unbuffered responses suitable for Comet and HTTP streaming applications
-    headers['X-Accel-Buffering'] = 'no'
+    headers["X-Accel-Buffering"] = "no"
     headers["Cache-Control"] ||= "no-cache"
+    headers["Last-Modified"] = Time.current.httpdate
     headers.delete("Content-Length")
   end
 

spec/controllers/admin/archived/petitions_controller_spec.rb

@@ -69,8 +69,9 @@
         end
 
         it "sets the streaming headers" do
-          expect(response["Cache-Control"]).to match(/no-cache/)
           expect(response["X-Accel-Buffering"]).to eq("no")
+          expect(response["Cache-Control"]).to match(/no-cache/)
+          expect(response["Last-Modified"]).to match(/\w{3}, \d{2} \w{3} \d{4} \d{2}:\d{2}:\d{2} GMT/)
         end
 
         it "sets the content disposition" do

spec/controllers/admin/petitions_controller_spec.rb

@@ -65,8 +65,9 @@
         end
 
         it "sets the streaming headers" do
-          expect(response["Cache-Control"]).to match(/no-cache/)
           expect(response["X-Accel-Buffering"]).to eq("no")
+          expect(response["Cache-Control"]).to match(/no-cache/)
+          expect(response["Last-Modified"]).to match(/\w{3}, \d{2} \w{3} \d{4} \d{2}:\d{2}:\d{2} GMT/)
         end
 
         it "sets the content disposition" do