Fixes the issue reported by @skyluc

This is a side-effect of the changes I've made in the partitioner in the previous commit
(I'll squash this commit with the previous one before merging).

My gut feeling is that the way we compute the dirty region should be improved, but I'd
rather do this in a separate PR.

Also, this seemingly innocuous change could lead to new issues in the Template editor,
because the PlayDocumentPartitioner is used by both the Route and the Template editors.
I've done some minimal monkey testing in the template editor after this change, and I haven't
seen any issue, but I can't state I'm confident nothing bad will happen out of this.