Fix "Test is missing assertions" in middleware_stack_proxy_test.rb

`Minitest::Mock#expect` does not increase a test's assertions count.
What I did in this PR is simply increment `self.assertions` when the
custom `#assert_playback` method is called.

Author: davidstosik

railties/test/configuration/middleware_stack_proxy_test.rb

@@ -63,18 +63,16 @@ def test_order
         @stack.swap :foo
         @stack.delete :foo
 
-        mock = Minitest::Mock.new
-        mock.expect :swap, nil, [:foo]
-        mock.expect :delete, nil, [:foo]
-
-        @stack.merge_into mock
-        mock.verify
+        assert_playback([:swap, :delete], :foo)
       end
 
       private
-        def assert_playback(msg_name, args)
+        def assert_playback(msg_names, args)
+          self.assertions += 1
           mock = Minitest::Mock.new
-          mock.expect msg_name, nil, [args]
+          Array(msg_names).each do |msg_name|
+            mock.expect msg_name, nil, [args]
+          end
           @stack.merge_into(mock)
           mock.verify
         end