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).

未指定 repository 檔案或測試。首先使用 Content-Type application/sparql-update 和 PREFIX 宣告重現 PATCH 請求，然後將 parser 的行為與 rdflib.js issue 651 進行比較；完成標準是接受有效的 PREFIX payload，同時對無效 @prefix 的處理遵循預期的 SPARQL 相容性。