First Attempt at fixing the CardDAV ressource to not ignore additional recurrence-ID VEVENTs as created by e.g. the default Android Calendar (Used instead of creating EXDATEs in the original event).
See RFC 4791, Chapter 4.1, especially the following exerpt:
Calendar components with the same UID property value, in a given
calendar collection, MUST be contained in the same calendar object
resource. This ensures that all components in a recurrence "set" are
contained in the same calendar object resource. It is possible for a
calendar object resource to just contain components that represent
"overridden" instances (ones that modify the behavior of a regular
instance, and thus include a RECURRENCE-ID property) without also
including the "master" recurring component (the one that defines the
recurrence "set" and does not contain any RECURRENCE-ID property).
Fetching Items works, however especially everything that is not multifetched is buggy when sending data back to the server. This is however just a first draft to get my changes out in the wild.