Make instances of HeaderValue and ContentType immutable

pkg/http/lib/src/multipart_request.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// 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 multipart_request;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';
 import 'dart:uri';
 import 'dart:utf';
@@ skipping 125 matching lines @@
   ];
 
   /// Returns the header string for a field. The return value is guaranteed to
   /// contain only ASCII characters.
   String _headerForField(String name, String value) {
     // http://tools.ietf.org/html/rfc2388 mandates some complex encodings for
     // field names and file names, but in practice user agents seem to just
     // URL-encode them so we do the same.
     var header = 'content-disposition: form-data; name="${encodeUri(name)}"';
     if (!isPlainAscii(value)) {
-      header = '$header\r\ncontent-type: text/plain; charset=UTF-8';
+      header = '$header\r\ncontent-type: text/plain; charset=utf-8';
     }
     return '$header\r\n\r\n';
   }
 
   /// Returns the header string for a file. The return value is guaranteed to
   /// contain only ASCII characters.
   String _headerForFile(MultipartFile file) {
     var header = 'content-type: ${file.contentType}\r\n'
       'content-disposition: form-data; name="${encodeUri(file.field)}"';
 
     if (file.filename != null) {
       header = '$header; filename="${encodeUri(file.filename)}"';
     }
     return '$header\r\n\r\n';
   }
 
   /// Returns a randomly-generated multipart boundary string of the given
   /// [length].
   String _boundaryString(int length) {
     var prefix = "dart-http-boundary-";
     var list = new List<int>(length - prefix.length);
     for (var i = 0; i < list.length; i++) {
       list[i] = _BOUNDARY_CHARACTERS[
           _random.nextInt(_BOUNDARY_CHARACTERS.length)];
     }
     return "$prefix${new String.fromCharCodes(list)}";
   }
 }