Use content negotiation to ask for machine readable data when getting urls/handles

[emanueldima]

VCR collections have a handle, but without an accept header the response mediatype is plain html. So when getting any url or handle in general, the switchboard should first ask for xml/cmdi/json data, and if that fails to ask for the resource without any accept header.

see also clarin-eric/VirtualCollectionRegistry#140 (useful for development).