Fix outdated BCF8 description and headers

Relevant section: 6.4.2.1.1 JSON parsing failure
https://graphql.github.io/graphql-over-http/draft/#sec-application-graphql-response-json.Examples.JSON-parsing-failure

Author: DoctorJohn

src/audits/server.ts

@@ -579,12 +579,13 @@ export function serverAudits(opts: ServerAuditOptions): Audit[] {
     ),
     audit(
       'BCF8',
-      'MAY use 400 status code on JSON parsing failure',
+      'SHOULD use 400 status code on JSON parsing failure when accepting application/json',
       async () => {
         const res = await fetchFn(await getUrl(opts.url), {
           method: 'POST',
           headers: {
             'content-type': 'application/json',
+            accept: 'application/json',
           },
           body: '{ "not a JSON',
         });

tests/__snapshots__/audits.test.ts.snap

@@ -192,7 +192,7 @@ exports[`should not change globally unique audit ids 1`] = `
   },
   {
     "id": "BCF8",
-    "name": "MAY use 400 status code on JSON parsing failure",
+    "name": "SHOULD use 400 status code on JSON parsing failure when accepting application/json",
   },
   {
     "id": "8764",