Use default Mashlib Accept header instead of */*

[RubenVerborgh]

The editor currently accepts any content type; as such, it might receive another content type (JSON-LD?) than other parts of Mashlib (typically Turtle).

The current Mashlib header is image/*;q=0.9, */*;q=0.1, application/rdf+xml;q=0.9, application/xhtml+xml, text/xml;q=0.5, application/xml;q=0.5, text/html;q=0.9, text/plain;q=0.5, text/n3;q=1.0, text/turtle;q=1.