Support HashWithIndifferentAccess#slice

Author: shuuuuun

lib/thor/core_ext/hash_with_indifferent_access.rb

@@ -38,6 +38,10 @@ def fetch(key, *args)
         super(convert_key(key), *args)
       end
 
+      def slice(*keys)
+        super(*keys.map{ convert_key(_1) })
+      end
+
       def key?(key)
         super(convert_key(key))
       end