Handle HTTP header parameters with empty values better

sdk/lib/io/http_headers.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.
 
 part of dart.io;
 
 class _HttpHeaders implements HttpHeaders {
   final Map<String, List<String>> _headers;
   final String protocolVersion;
 
@@ skipping 742 matching lines @@
         skipWS();
         if (done()) return;
         String name = parseParameterName();
         skipWS();
         if (done()) {
           parameters[name] = null;
           return;
         }
         maybeExpect("=");
         skipWS();
-        if(done()) {
+        if (done()) {
           parameters[name] = null;
           return;
         }
         String value = parseParameterValue();
-        if (name == 'charset' && this is _ContentType) {
+        if (name == 'charset' && this is _ContentType && value != null) {
           // Charset parameter of ContentTypes are always lower-case.
           value = value.toLowerCase();
         }
         parameters[name] = value;
         skipWS();
         if (done()) return;
         // TODO: Implement support for multi-valued parameters.
         if(s[index] == valueSeparator) return;
         expect(parameterSeparator);
       }
@@ skipping 211 matching lines @@
             (codeUnit >= 0x23 && codeUnit <= 0x2B) ||
             (codeUnit >= 0x2D && codeUnit <= 0x3A) ||
             (codeUnit >= 0x3C && codeUnit <= 0x5B) ||
             (codeUnit >= 0x5D && codeUnit <= 0x7E))) {
         throw new FormatException(
             "Invalid character in cookie value, code unit: '$codeUnit'");
       }
     }
   }
 }