Make BaseRequest.contentType use null rather than -1 as a flag value.

pkg/http/test/mock_client_test.dart

 // Copyright (c) 2012, 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 mock_client_test;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:http/http.dart' as http;
@@ skipping 22 matching lines @@
 
   test('handles a streamed request', () {
     var client = new MockClient.streaming((request, bodyStream) {
       return bodyStream.bytesToString().then((bodyString) {
         var controller = new StreamController<List<int>>(sync: true);
         async.then((_) {
           controller.add('Request body was "$bodyString"'.codeUnits);
           controller.close();
         });
 
-        return new http.StreamedResponse(controller.stream, 200, -1);
+        return new http.StreamedResponse(controller.stream, 200);
       });
     });
 
     var uri = Uri.parse("http://example.com/foo");
     var request = new http.Request("POST", uri);
     request.body = "hello, world";
     var future = client.send(request)
         .then(http.Response.fromStream)
         .then((response) => response.body);
     expect(future, completion(equals('Request body was "hello, world"')));
   });
 
   test('handles a request with no body', () {
     var client = new MockClient((request) {
       return new Future.value(new http.Response('you did it', 200));
     });
 
     expect(client.read("http://example.com/foo"),
         completion(equals('you did it')));
   });
 }