Allow JSON requests for petitions before opening

It's up to the consumer of the data to respond appropriately.

app/controllers/petitions_controller.rb

@@ -102,7 +102,9 @@ def redirect_to_home_page_if_dissolved
   end
 
   def redirect_to_home_page_unless_opened
-    redirect_to home_url unless Parliament.opened?
+    unless json_request?
+      redirect_to home_url unless Parliament.opened?
+    end
   end
 
   def request_format

spec/controllers/petitions_controller_spec.rb

@@ -501,6 +501,11 @@
         get :index
         expect(response).to redirect_to("https://petition.parliament.uk/")
       end
+
+      it "doesn't redirect JSON requests" do
+        get :index, as: :json
+        expect(response).to be_successful
+      end
     end
   end