Set type when creating mappings in specs

Mapping.type can't be null, so we need to set it to something in
test setup. Even though we're not using the new type column yet, we
might as well set it to the planned values already rather than '' or
something nonsense.

The only supported mapping types are 'redirect' and 'archive' at this
point. One test creates a mapping with an http_status of '404', and
I've given this the type 'never_existed' - this shouldn't be taken as
a decision about what this type should be called if we ever decide to
support it in Transition.