Fix ActiveModel::Type::DateTime#serialize

`ActiveModel::Type::DateTime#serialize` should return a `Time` object
so that finding by a datetime column works correctly.

activemodel/lib/active_model/type/date_time.rb

@@ -10,6 +10,10 @@ def type
         :datetime
       end
 
+      def serialize(value)
+        super(cast(value))
+      end
+
       private
 
         def cast_value(value)

activerecord/test/cases/date_time_test.rb

@@ -58,4 +58,17 @@ def test_assign_in_local_timezone
       assert_equal now, task.starting
     end
   end
+
+  def test_date_time_with_string_value_with_subsecond_precision
+    skip unless subsecond_precision_supported?
+    string_value = "2017-07-04 14:19:00.5"
+    topic = Topic.create(written_on: string_value)
+    assert_equal topic, Topic.find_by(written_on: string_value)
+  end
+
+  def test_date_time_with_string_value_with_non_iso_format
+    string_value = "04/07/2017 2:19pm"
+    topic = Topic.create(written_on: string_value)
+    assert_equal topic, Topic.find_by(written_on: string_value)
+  end
 end