Add Headers::qvalues() to implement content negotiation

See https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation

src/main/php/web/Headers.class.php

@@ -95,6 +95,40 @@ protected function next($input, &$offset) {
     };
   }
 
+  /**
+   * Returns a new parser for qvalues headers, e.g.:
+   *
+   * `Accept-Encoding: deflate, gzip;q=1.0, *;q=0.5`
+   *
+   * @return self
+   */
+  public static function qvalues() {
+    return new class() extends Headers {
+      protected function next($input, &$offset) {
+        $weighted= [];
+        $q= 1.0;
+        do {
+          $s= strcspn($input, ',;', $offset);
+          $value= ltrim(substr($input, $offset, $s), ' ');
+          $offset+= $s + 1;
+
+          $c= $input[$offset - 1] ?? null;
+          if (';' === $c) {
+            $weighted[$value]= (float)(Headers::pairs()->next($input, $offset)['q'] ?? $q);
+            $c= $input[$offset - 1] ?? null;
+          } else {
+            $weighted[$value]= $q;
+          }
+
+          $q-= 0.0001;
+        } while ($c);
+
+        arsort($weighted);
+        return $weighted;
+      }
+    };
+  }
+
   /**
    * Returns a new parser for headers with key/value pairs
    *

src/test/php/web/unittest/HeadersTest.class.php

@@ -45,12 +45,16 @@ public function refresh($header) {
   #[Test]
   public function accept() {
     Assert::equals(
-      [
-        new Parameterized('text/html', []),
-        new Parameterized('application/json', ['q' => '0.9']),
-        new Parameterized('*/*', ['q' => '0.8']),
-      ],
-      Headers::values(Headers::parameterized())->parse('text/html, application/json;q=0.9, */*;q=0.8')
+      ['text/html' => 1.0, 'application/json' => 0.9, '*/*' => 0.8],
+      Headers::qvalues()->parse('text/html, application/json;q=0.9, */*;q=0.8')
+    );
+  }
+
+  #[Test]
+  public function accept_encoding() {
+    Assert::equals(
+      ['deflate' => 1.0, 'gzip' => 1.0, '*' => 0.5],
+      Headers::qvalues()->parse('deflate, gzip;q=1.0, *;q=0.5')
     );
   }