Fix event type when workflow is canceled (1st task of the workflow was rejected)

When the 1st task of a serial workflow is rejected, the document goes back to the iniitiator, who has two actions available: resubmit the document to the workflow and Cancel the workflow. When he clicks the Cancel workflow button, the event tracked in the document History is "Workflow completed". It should be "Workflow canceled" (this event already exists, when user clicks on the Abandon button on the Workflow tab).