ActiveRecord::Coders::YAMLColumn#dump should raise an error

closes #2737

Conflicts:

	activerecord/lib/active_record/coders/yaml_column.rb

activerecord/lib/active_record/coders/yaml_column.rb

@@ -15,7 +15,13 @@ module ActiveRecord
       end
 
       def dump(obj)
-        YAML.dump(obj) unless obj.nil?
+        return if obj.nil?
+
+        unless obj.is_a?(object_class)
+          raise SerializationTypeMismatch,
+            "Attribute was supposed to be a #{object_class}, but was a #{obj.class}. -- #{obj.inspect}"
+        end
+        YAML.dump obj
       end
 
       def load(yaml)

activerecord/test/cases/coders/yaml_column_test.rb

@@ -9,6 +9,13 @@ module ActiveRecord
         assert_equal Object, coder.object_class
       end
 
+      def test_type_mismatch_on_different_classes_on_dump
+        coder = YAMLColumn.new(Array)
+        assert_raises(SerializationTypeMismatch) do
+          coder.dump("a")
+        end
+      end
+
       def test_type_mismatch_on_different_classes
         coder = YAMLColumn.new(Array)
         assert_raises(SerializationTypeMismatch) do