Fix conformance to SPARQL Update / Query's prefix declaration

Snippet of an example request:

PATCH https://example.org/test.ttl  HTTP/1.1
Content-Type: application/sparql-update

PREFIX acl: <http://www.w3.org/ns/auth/acl#>

Snippet of an example response:

HTTP/1.1 400 Bad Request
X-Powered-By: solid-server/5.7.11
Accept-Patch: application/sparql-update
Content-Type: text/plain; charset=utf-8
Content-Length: 206

Patch document syntax error: Line 1 of <https://example.org/test.ttl>: Bad syntax:
   Unknown syntax at start of statememt: 'PREFIX acl: <http://'/
   at: "PREFIX acl: <http://www.w3.org/"

AFAICT, https://github.com/linkeddata/rdflib.js/issues/651 captures the issue in that NSS rejects SPARQL Update requests with valid payload using PREFIX (and accepts invalid payload containing @prefix).

Non viene indicato alcun file del repository né alcun test. Inizia riproducendo la richiesta PATCH con Content-Type application/sparql-update e la dichiarazione PREFIX, quindi confronta il comportamento del parser con rdflib.js issue 651; il lavoro è completato quando i payload PREFIX validi vengono accettati, mentre la gestione di @prefix non valido segue la conformità SPARQL prevista.