Format Date in ISO-8601 explicitly

Fix #644

Author: nobu

lib/psych/visitors/yaml_tree.rb

@@ -189,7 +189,8 @@ def visit_Regexp o
       end
 
       def visit_Date o
-        register o, visit_Integer(o.gregorian)
+        formatted = format_date o
+        register o, @emitter.scalar(formatted, nil, nil, true, false, Nodes::Scalar::ANY)
       end
 
       def visit_DateTime o
@@ -486,6 +487,10 @@ def format_time time, utc = time.utc?
         end
       end
 
+      def format_date date
+        date.strftime("%Y-%m-%d")
+      end
+
       def register target, yaml_obj
         @st.register target, yaml_obj
         yaml_obj

test/psych/test_date_time.rb

@@ -85,5 +85,20 @@ def test_alias_with_time
       assert_match('&', yaml)
       assert_match('*', yaml)
     end
+
+    def test_overwritten_to_s
+      pend "Failing on JRuby" if RUBY_PLATFORM =~ /java/
+      s = Psych.dump(Date.new(2023, 9, 2), permitted_classes: [Date])
+      assert_separately(%W[-rpsych -rdate - #{s}], "#{<<~"begin;"}\n#{<<~'end;'}")
+      class Date
+        undef to_s
+        def to_s; strftime("%D"); end
+      end
+      expected = ARGV.shift
+      begin;
+        s = Psych.dump(Date.new(2023, 9, 2), permitted_classes: [Date])
+        assert_equal(expected, s)
+      end;
+    end
   end
 end