Add a test for mutating times

Author: jhawthorn

activerecord/test/cases/base_test.rb

@@ -1172,6 +1172,16 @@ def test_switching_default_time_zone
       end
     end
 
+    def test_mutating_time_objects
+      with_env_tz do
+        with_timezone_config default: :local do
+          assert_equal Time.local(2004, 1, 1, 0, 0, 0, 0), Default.new.fixed_time
+          assert_equal Time.utc(2004, 1, 1, 5, 0, 0, 0), Default.new.fixed_time.utc
+          assert_equal Time.local(2004, 1, 1, 0, 0, 0, 0), Default.new.fixed_time
+        end
+      end
+    end
+
     unless in_memory_db?
       def test_connection_in_local_time
         with_timezone_config default: :utc do