read_only dav storage still creates a collection

I am playing around with vdirsyncer==0.16.8 and I intended to have set up my DAV backend as read_only, because I am not yet ready to let this program modify things on my server. So I configured read_only = true in my caldav storage. Because I couldn't make the integration with khal work (https://github.com/pimutils/khal/issues/991) I played around with the collections parameter which I've changed from null to ["collection"]. When running vdirsyncer discover it prompted me:

warning: No collection "tmms" found for storage baikal_tmms.
Should vdirsyncer attempt to create it? [y/N]: y

I accidentally hit "y". Mea culpa. But now I have a new collection on my read_only storage.
I expected the program to not modify my server, e.g. to error out when it encounters that the collection does not exist and the backend is configured as read-only. Bonuspoints if it shows me how print the existing collections and their capabilities to choose from.

Start at the vdirsyncer discover entry point and trace how a missing collection is handled when the storage is configured with read_only = true. Confirm the behavior against a read-only DAV storage and ensure discovery does not create a collection; the existing-collections and capabilities display is an optional follow-up.