I seem to have found a bug bug relating to date fields when other fields
on a form a invalid.

How to (I believe) repoduce:
- Have a node type that has a form with a date field
- Have an auditor for the same node that checks certain values and
raises a 'ValueError' if there is some problem

- Create or edit the node (ensuring there is no existing value for the
date field)
- Enter a date into the date field
- Enter a value into another fieldf that will cause a 'ValueError'
- Submit the form

The process should fail because the template engine(?) treats the date
value as a 'Date', when instead it is still a 'String'. See attached
traceback.