IMAP: switch to GID-based merge when the Collection can contain something else than emails

In order to fix the recurrent multiple-merge-candidates issue which was breaking
ItemSync, ItemSync switched to RID-based merging, which is way more reliable.
However in some cases the IMAP resource still wants to use GID-based merging,
because RID might not be stable enough.

(cherry picked from commit 93a2baac)