fix object grow

and write a test (and another one …)

test/parfait/test_object.rb

@@ -10,6 +10,11 @@ class TestObject < MiniTest::Test
     assert_equal 0 ,  @object.internal_object_length
   end
 
+  def test_list_grow
+    @object.internal_object_grow( 2 )
+    assert_equal 2 ,  @object.internal_object_length
+  end
+
   def test_empty_list_doesnt_return
     assert_equal nil ,  @object.internal_object_get(3)
   end