Fixed losing of flash object in redirect_to_finish_wizard.

When the wizard was redirecting to the finish path (defined in the controller), it was making two redirects:
* first to the `wicked_finish` step
* then to the defined by the user path

In the process the options hasht that was passed to the `render_wizard` method call (`render_wizard @resoource, options`), specifically the flash object, was lost.

Calling the `flash.keep` method prevents rails from flushing the flash object for one more request.