Make instances of HeaderValue and ContentType immutable

pkg/http/lib/src/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 request;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:scalarlist';
 import 'dart:uri';
 
@@ skipping 38 matching lines @@
       return _defaultEncoding;
     }
     return requiredEncodingForCharset(_contentType.charset);
   }
 
   set encoding(Encoding value) {
     _checkFinalized();
     _defaultEncoding = value;
     var contentType = _contentType;
     if (contentType != null) {
-      contentType.charset = value.name;
+      contentType = new ContentType(contentType.primaryType,
+                                    contentType.subType,
+                                    charset: value.name,
+                                    parameters: contentType.parameters);
       _contentType = contentType;
     }
   }
 
   // TODO(nweiz): make this return a read-only view
   /// The bytes comprising the body of the request. This is converted to and
   /// from [body] using [encoding].
   ///
   /// This list should only be set, not be modified in place.
   Uint8List get bodyBytes => _bodyBytes;
   Uint8List _bodyBytes;
 
   set bodyBytes(List<int> value) {
     _checkFinalized();
     _bodyBytes = toUint8List(value);
   }
 
   /// The body of the request as a string. This is converted to and from
   /// [bodyBytes] using [encoding].
   ///
   /// When this is set, if the request does not yet have a `Content-Type`
   /// header, one will be added with the type `text/plain`. Then the `charset`
   /// parameter of the `Content-Type` header (whether new or pre-existing) will
   /// be set to [encoding] if it wasn't already set.
   String get body => decodeString(bodyBytes, encoding);
 
   set body(String value) {
     bodyBytes = encodeString(value, encoding);
     var contentType = _contentType;
-    if (contentType == null) contentType = new ContentType("text", "plain");
-    if (contentType.charset == null) contentType.charset = encoding.name;
+    if (contentType == null) {
+      contentType = new ContentType("text", "plain", charset: encoding.name);
+    } else if (contentType.charset == null) {
+      contentType = new ContentType(contentType.primaryType,
+                                    contentType.subType,
+                                    charset: encoding.name,
+                                    parameters: contentType.parameters);
+    }
     _contentType = contentType;
   }
 
   /// The form-encoded fields in the body of the request as a map from field
   /// names to values. The form-encoded body is converted to and from
   /// [bodyBytes] using [encoding] (in the same way as [body]).
   ///
   /// If the request doesn't have a `Content-Type` header of
   /// `application/x-www-form-urlencoded`, reading this will throw a
   /// [StateError].
@@ skipping 49 matching lines @@
   set _contentType(ContentType value) {
     headers[HttpHeaders.CONTENT_TYPE] = value.toString();
   }
 
   /// Throw an error if this request has been finalized.
   void _checkFinalized() {
     if (!finalized) return;
     throw new StateError("Can't modify a finalized Request.");
   }
 }