Validates response processed by exception handler

If block of `rescue_from` return nil, the response would be nil.
That causes system information leak. Add a validation to prevent it.

lib/grape/middleware/error.rb

@@ -127,7 +127,13 @@ def run_rescue_handler(handler, error)
           handler = public_method(handler)
         end
 
-        handler.arity.zero? ? instance_exec(&handler) : instance_exec(error, &handler)
+        response = handler.arity.zero? ? instance_exec(&handler) : instance_exec(error, &handler)
+        valid_response?(response) ? response : error!('Internal Server Error(Invalid Response)')
+      end
+
+      def valid_response?(response)
+        # Rack::Response.new(...).finish generates an array with size 3
+        response.is_a?(Array) && response.size == 3
       end
     end
   end

spec/grape/api_spec.rb

@@ -1723,6 +1723,26 @@ class CustomError < Grape::Exceptions::Base; end
       expect(last_response.status).to eql 500
       expect(last_response.body).to eq('Formatter Error')
     end
+
+    it 'validates response processed by exception handler' do
+      subject.rescue_from ArgumentError do
+        error!('rain!')
+        nil # invalid response caused by return nil
+      end
+      subject.rescue_from :all do
+        error!('rain!')
+      end
+      subject.get('/invalid_response') { raise ArgumentError }
+      subject.get('/valid_response') { raise 'rain!' }
+
+      get '/invalid_response'
+      expect(last_response.status).to eql 500
+      expect(last_response.body).to eq('Internal Server Error(Invalid Response)')
+
+      get '/valid_response'
+      expect(last_response.status).to eql 500
+      expect(last_response.body).to eq('rain!')
+    end
   end
 
   describe '.rescue_from klass, block' do