Use be_not_found instead of be_missing

We were getting a deprecation message in Rails 5.2:

The missing? predicate is deprecated and will be removed in Rails 6.0.
Please use not_found? as provided by Rack::Response::Helpers

spec/controllers/pages_controller_spec.rb

@@ -38,12 +38,12 @@ describe PagesController do
   describe "Not found pages" do
     it "returns a 404 message" do
       get :show, params: { id: "nonExistentPage" }
-      expect(response).to be_missing
+      expect(response).to be_not_found
     end
 
     it "returns a 404 message for a JavaScript request" do
       get :show, params: { id: "nonExistentJavaScript.js" }
-      expect(response).to be_missing
+      expect(response).to be_not_found
     end
   end
 end