Fixed update_attributeS issue

Hi,

as reported in <a href="https://github.com/geekq/workflow/issues/57">Issue #57</a>, adding ```update_attributes``` instead of  ```update_attribute``` causes problems related with mass assignment.
You can either add workflow_state to attr_accessible (which is inacceptable for me), use update_attributes with ```without_protection``` attribute (which skips mass assignment protection), or use update_column method (which skips validations, callbacks etc) – I don’t have enough time to dig in workflows code, to decide which one fits here better, so please review this before merging.