Include user defined attributes in inspect

The fact that this only includes column names is an oversight.

activerecord/lib/active_record/core.rb

@@ -500,7 +500,7 @@ module ActiveRecord
       # We check defined?(@attributes) not to issue warnings if the object is
       # allocated but not initialized.
       inspection = if defined?(@attributes) && @attributes
-        self.class.column_names.collect do |name|
+        self.class.attribute_names.collect do |name|
           if has_attribute?(name)
             "#{name}: #{attribute_for_inspect(name)}"
           end

activerecord/test/cases/attributes_test.rb

@@ -249,5 +249,11 @@ module ActiveRecord
       model.foo = "lol"
       refute model.changed?
     end
+
+    test "attributes not backed by database columns appear in inspect" do
+      inspection = OverloadedType.new.inspect
+
+      assert inspection.include?("non_existent_decimal")
+    end
   end
 end