Clean out teaser on re-import

If you import a file to a new node, and the teaser is not filled.... then Drupal Core apparently breaks off the first chunk of the body and saves it in the `teaser` database field.
If you re-import the same file, and the teaser is not filled... then Drupal Core does not reset the contents `teaser` database field.

Effect: The node shows fine. But at the first edit of this re-imported node, the old teaser is suddenly prepended to the node body!

Solution: clean out the teaser. See patch.