HeaderValues: Handle all parameters without values.

tests/standalone/io/http_headers_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart.io;
 
 import "package:expect/expect.dart";
 import "dart:async";
 import "dart:collection";
 import "dart:convert";
@@ skipping 278 matching lines @@
       "filename": "genome.jpeg",
       "modification-date": "Wed, 12 February 1997 16:29:51 -0500"
   };
   check(headerValue, "attachment", parameters);
   headerValue = new HeaderValue("attachment", parameters);
   check(headerValue, "attachment", parameters);
   headerValue = HeaderValue.parse(
     "  attachment  ;filename=genome.jpeg  ;"
     "modification-date = \"Wed, 12 February 1997 16:29:51 -0500\""  );
   check(headerValue, "attachment", parameters);
+  headerValue = HeaderValue.parse("xxx; aaa; bbb; ccc");
+  check(headerValue, "xxx", {"aaa": null, "bbb": null, "ccc": null});
 }
 
 void testContentType() {
   void check(ContentType contentType,
              String primaryType,
              String subType,
              [Map parameters]) {
     Expect.equals(primaryType, contentType.primaryType);
     Expect.equals(subType, contentType.subType);
     Expect.equals("$primaryType/$subType", contentType.value);
@@ skipping 260 matching lines @@
   testHeaderValue();
   testContentType();
   testContentTypeCache();
   testCookie();
   testInvalidCookie();
   testHeaderLists();
   testInvalidFieldName();
   testInvalidFieldValue();
   testClear();
 }