Refactor join_collection
Refactor join_collection

See #194

untitaker/vdirsyncer,tribut/vdirsyncer,untitaker/vdirsyncer,credativUK/vdirsyncer,tribut/vdirsyncer,hobarrera/vdirsyncer,credativUK/vdirsyncer,hobarrera/vdirsyncer,untitaker/vdirsyncer