Only lower-case ContentType's values if the name is 'charset'.

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 314 matching lines @@
   check(contentType, "text", "html");
   Expect.equals("text/html", contentType.toString());
   contentType = new ContentType("text", "html", charset: "utf-8");
   check(contentType, "text", "html", {"charset": "utf-8"});
   Expect.equals("text/html; charset=utf-8", contentType.toString());
   Expect.throws(() => contentType.parameters["xxx"] = "yyy",
                 (e) => e is UnsupportedError);
 
   contentType = new ContentType("text",
                                 "html",
-                                parameters: {"CHARSET": "UTF-8", "xxx": "yyy"});
-  check(contentType, "text", "html", {"charset": "utf-8", "xxx": "yyy"});
+                                parameters: {"CHARSET": "UTF-8", "xxx": "YYY"});
+  check(contentType, "text", "html", {"charset": "utf-8", "xxx": "YYY"});
   String s = contentType.toString();
-  bool expectedToString = (s == "text/html; charset=utf-8; xxx=yyy" ||
-                           s == "text/html; xxx=yyy; charset=utf-8");
+  bool expectedToString = (s == "text/html; charset=utf-8; xxx=YYY" ||
+                           s == "text/html; xxx=YYY; charset=utf-8");
   Expect.isTrue(expectedToString);
+  contentType = ContentType.parse("text/html; CHARSET=UTF-8; xxx=YYY");
+  check(contentType, "text", "html", {"charset": "utf-8", "xxx": "YYY"});
   Expect.throws(() => contentType.parameters["xxx"] = "yyy",
                 (e) => e is UnsupportedError);
 
   contentType = new ContentType("text",
                                 "html",
                                 charset: "ISO-8859-1",
                                 parameters: {"CHARSET": "UTF-8", "xxx": "yyy"});
   check(contentType, "text", "html", {"charset": "iso-8859-1", "xxx": "yyy"});
   s = contentType.toString();
   expectedToString = (s == "text/html; charset=iso-8859-1; xxx=yyy" ||
@@ skipping 213 matching lines @@
   testHeaderValue();
   testContentType();
   testContentTypeCache();
   testCookie();
   testInvalidCookie();
   testHeaderLists();
   testInvalidFieldName();
   testInvalidFieldValue();
   testClear();
 }