Organizations becoming Individuals during import

bug reported by Dave Jenkins:

(1) Import some Orgs
(2) Import some Individuals (specifying Fill), with field mapping specifying that the org name field in csv maps to "Contact for" Organization Name. "Contact for" is defined from Individual to Organization.

Result: import worked but approx half of my Organizations got
transformed into Individuals - which presumably should never happen -
where new Individuals should have been created instead. I think the
ones that got transformed are where the Individual's email address
matched an existing Organization's, which happened a lot because
they're the contacts for those Orgs. So I'm guessing there are bugs
whereby (a) the import is not checking contact_type when looking for
matches, and (b) contact_type figures in an update.