Add a test for bevhavior of returning Rack response by after callback

Author: namusyaka

spec/grape/middleware/base_spec.rb

@@ -38,6 +38,16 @@
     end
   end
 
+  context 'after callback' do
+    before do
+      allow(subject).to receive(:after).and_return([200, {}, 'Hello from after callback'])
+    end
+
+    it 'overwrites application response' do
+      expect(subject.call!({}).last).to eq('Hello from after callback')
+    end
+  end
+
   it 'is able to access the response' do
     subject.call({})
     expect(subject.response).to be_kind_of(Rack::Response)