Spec 3.9: reject creation of resources with external content

[scossu]

External content handling is not supported at the moment and, as per 3.9, must be explicitly rejected:

Fedora servers that do not support the creation of LDP-NRs with content external to the request entity MUST reject such requests with a 4xx range status code, and MUST describe this restriction in a resource indicated by a rel="http://www.w3.org/ns/ldp#constrainedBy" link in the Link response header.

External content may be implemented later, in which case it should be enabled by configuration (i.e. leave the implementer the choice to allow or forbid such content).