Add custom namespaces definitions

[Anatoliy057]

I'm trying to make my caldav implementation based on this webdav server implementation and ran into the problem that I can't add an alias for the urn:ietf:params:xml:ns:caldav namespace. I understand that with the current implementation the alias is created automatically and it does not affect functionality, but for testing it is more convenient when the expected response will be more understandable

[mar10]

Thanks for contributing! I will review it shortly