Turns out that this issue is not related to displaying the date using
field(), but attempting to display the date in the history section of
the issue. When generating the "current state" the DateHTMLProperty
._value contains a str instead of Date which causes problems when
calling .plain()

The actual issue is therefore: values that have not been committed to
the database (due to an auditor error) are showing up in this history
when an auditor error occurs.

To reproduce this subtle issue (in a visual way):
 * Create a new issue that is assigned to someone ("demo")
 * Edit the issue, and un-assign it from anyone ("- no selection -")
 * Edit the issue once again, and:
    * assign the issue to someone ("demo")
    * delete the contents of the title (to trigger an auditor error)

The history should look like:
  2012-03-06 09:28:06	demo	set	assignedto: demo -> 
  2012-03-06 09:27:54	demo	create	

But instead it looks like:
  2012-03-06 09:28:06	demo	set	assignedto: demo -> demo
  2012-03-06 09:27:54	demo	create