Merge pull request #8392 from reitermarkus/artifact-summarize

Add missing `summarize` method for `StageOnly`.

Library/Homebrew/cask/artifact/stage_only.rb

@@ -18,6 +18,10 @@ module Cask
       def to_a
         [true]
       end
+
+      def summarize
+        "true"
+      end
     end
   end
 end