Support more kindly for after method

lib/grape/middleware/base.rb

@@ -27,7 +27,7 @@ def call!(env)
         ensure
           after_response = after
         end
-        after_response || @app_response
+        response_valid?(after_response) ? after_response : @app_response
       end
 
       # @abstract
@@ -65,6 +65,17 @@ def mime_types
         end
         types_without_params
       end
+
+      def response_valid?(response)
+        if response.is_a?(Array)
+          status, headers, body = *response
+          (status >= 100 && status < 600) && (headers.is_a?(Hash)) && body.respond_to?(:each)
+        else
+          response.is_a?(Rack::Response)
+        end
+      rescue
+        false
+      end
     end
   end
 end

spec/grape/middleware/base_spec.rb

@@ -40,11 +40,21 @@
 
   context 'after callback' do
     before do
-      allow(subject).to receive(:after).and_return([200, {}, 'Hello from after callback'])
+      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')
+      expect(subject.call!({}).last).to eq(['Hello from after callback'])
+    end
+  end
+
+  context 'after callback with invalid response' do
+    before do
+      allow(subject).to receive(:after).and_return('invalid response')
+    end
+
+    it 'does not overwrite application response' do
+      expect(subject.call!({}).last).to eq('Hi there.')
     end
   end