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

pkg/http/lib/src/response.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 response;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:typed_data';
@@ skipping 39 matching lines @@
       List<int> bodyBytes,
       int statusCode,
       {BaseRequest request,
        Map<String, String> headers: const {},
        bool isRedirect: false,
        bool persistentConnection: true,
        String reasonPhrase})
     : bodyBytes = toUint8List(bodyBytes),
       super(
         statusCode,
-        bodyBytes.length,
+        contentLength: bodyBytes.length,
         request: request,
         headers: headers,
         isRedirect: isRedirect,
         persistentConnection: persistentConnection,
         reasonPhrase: reasonPhrase);
 
   /// Creates a new HTTP response by waiting for the full body to become
   /// available from a [StreamedResponse].
   static Future<Response> fromStream(StreamedResponse response) {
     return response.stream.toBytes().then((body) {
@@ skipping 15 matching lines @@
 Encoding _encodingForHeaders(Map<String, String> headers) =>
   encodingForCharset(_contentTypeForHeaders(headers).charset);
 
 /// Returns the [ContentType] object for the given headers. Defaults to
 /// `application/octet-stream`.
 ContentType _contentTypeForHeaders(Map<String, String> headers) {
   var contentType = headers[HttpHeaders.CONTENT_TYPE];
   if (contentType != null) return ContentType.parse(contentType);
   return new ContentType("application", "octet-stream");
 }