Content type for SPARQL update queries is overridden with a warning

[lkitching]

The explicit application/sparql-update content type being set for SPARQL update requests is being overridden by RestClient to with a logged warning:

warning: Overriding "Content-Type" header "application/sparql-update" with "application/x-www-form-urlencoded" due to payload

According to the SPARQL update protocol (https://www.w3.org/TR/sparql11-protocol/#query-via-post-urlencoded), the correct content type for update queries is application/x-www-form-urlencoded so remove the explicit content type.