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

pkg/http/test/streamed_request_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 streamed_request_test;
 
+import 'dart:convert';
+
 import 'package:http/http.dart' as http;
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 void main() {
-  test('#finalize freezes contentLength', () {
-    var request = new http.StreamedRequest('POST', dummyUrl);
-    request.finalize();
+  group('contentLength', () {
+    test('defaults to null', () {
+      var request = new http.StreamedRequest('POST', dummyUrl);
+      expect(request.contentLength, isNull);
+    });
 
-    expect(request.contentLength, equals(-1));
-    expect(() => request.contentLength = 10, throwsStateError);
+    test('disallows negative values', () {
+      var request = new http.StreamedRequest('POST', dummyUrl);
+      expect(() => request.contentLength = -1, throwsArgumentError);
+    });
+
+    test('controls the Content-Length header', () {
+      return startServer().then((_) {
+        var request = new http.StreamedRequest('POST', serverUrl);
+        request.contentLength = 10;
+        request.sink.add([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
+        request.sink.close();
+
+        return request.send();
+      }).then((response) {
+        expect(UTF8.decodeStream(response.stream),
+            completion(parse(containsPair('headers',
+                containsPair('content-length', ['10'])))));
+      }).whenComplete(stopServer);
+    });
+
+    test('defaults to sending no Content-Length', () {
+      return startServer().then((_) {
+        var request = new http.StreamedRequest('POST', serverUrl);
+        request.sink.add([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
+        request.sink.close();
+
+        return request.send();
+      }).then((response) {
+        expect(UTF8.decodeStream(response.stream),
+            completion(parse(containsPair('headers',
+                isNot(contains('content-length'))))));
+      }).whenComplete(stopServer);
+    });
+
+    test('is frozen by finalize()', () {
+      var request = new http.StreamedRequest('POST', dummyUrl);
+      request.finalize();
+      expect(() => request.contentLength = 10, throwsStateError);
+    });
   });
 }