Make instances of HeaderValue and ContentType immutable

pkg/http/test/http_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 http_test;
 
 import 'dart:io';
 
 import 'package:unittest/unittest.dart';
 import '../lib/http.dart' as http;
@@ skipping 40 matching lines @@
         }, fields: {
           'some-field': 'value',
           'other-field': 'other value'
         }).then((response) {
           expect(response.statusCode, equals(200));
           expect(response.body, parse(equals({
             'method': 'POST',
             'path': '/',
             'headers': {
               'content-type': [
-                'application/x-www-form-urlencoded; charset=UTF-8'
+                'application/x-www-form-urlencoded; charset=utf-8'
               ],
               'content-length': ['40'],
               'x-random-header': ['Value'],
               'x-other-header': ['Other Value']
             },
             'body': 'some-field=value&other-field=other+value'
           })));
         }), completes);
       }), completes);
     });
@@ skipping 28 matching lines @@
         }, fields: {
           'some-field': 'value',
           'other-field': 'other value'
         }).then((response) {
           expect(response.statusCode, equals(200));
           expect(response.body, parse(equals({
             'method': 'PUT',
             'path': '/',
             'headers': {
               'content-type': [
-                'application/x-www-form-urlencoded; charset=UTF-8'
+                'application/x-www-form-urlencoded; charset=utf-8'
               ],
               'content-length': ['40'],
               'x-random-header': ['Value'],
               'x-other-header': ['Other Value']
             },
             'body': 'some-field=value&other-field=other+value'
           })));
         }), completes);
       }), completes);
     });
@@ skipping 82 matching lines @@
       }), completes);
     });
 
     test('readBytes throws an error for a 4** status code', () {
       expect(startServer().then((_) {
         expect(http.readBytes(serverUrl.resolve('/error')), throwsHttpException);
       }), completes);
     });
   });
 }