Parse and sort accept headers; pick best q.

This commit uses code generated by a Google AI Overview result. They cite
no sources and claim no license. However, it is exactly what I was looking
for with that search.

Author: BigBlueHat

functions/test-suite/_middleware.js

@@ -1,11 +1,46 @@
+function parseAcceptHeader(acceptHeader) {
+  // generated by Google AI "parse accept header javascript"
+  if (!acceptHeader) {
+    return [];
+  }
+
+  return acceptHeader.split(',')
+    .map(item => item.trim())
+    .map(item => {
+      const parts = item.split(';');
+      const value = parts[0].trim();
+      let quality = 1;
+
+      if (parts.length > 1) {
+        const q = parts[1].trim();
+        if (q.startsWith('q=')) {
+          quality = parseFloat(q.substring(2));
+        }
+      }
+
+      return { value, quality };
+    })
+    .sort((a, b) => b.quality - a.quality);
+    // From:
+    // text/html, application/xhtml+xml, application/xml;q=0.9, */*;q=0.8
+    // Expected output:
+    // [
+    //   { value: 'text/html', quality: 1 },
+    //   { value: 'application/xhtml+xml', quality: 1 },
+    //   { value: 'application/xml', quality: 0.9 },
+    //   { value: '*/*', quality: 0.8 }
+    // ]
+}
+
 const typeToExt = {
   "text/turtle": "ttl",
   "application/ld+json": "jsonld"
 };
 
 export async function onRequest(context) {
   try {
-    const accept = context.request.headers.get('Accept');
+    const parsedAccept = parseAcceptHeader(context.request.headers.get('Accept'));
+    const accept = parsedAccept[0].value;
 
     // if we have a mapping for this extension, send it. Otherwise fall through.
     if(Object.keys(typeToExt).indexOf(accept) > -1) {