Add test for setting content-length through 'headers.set'.

tests/standalone/io/http_content_length_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.
 
 import "dart:isolate";
 import "dart:io";
 
 void testNoBody(int totalConnections, bool explicitContentLength) {
   var errors = 0;
   HttpServer.bind("127.0.0.1", 0, totalConnections).then((server) {
@@ skipping 213 matching lines @@
       socket.listen(
           (d) { },
           onDone: () {
             socket.destroy();
             server.close();
           });
     });
   });
 }
 
+void testSetContentLength() {
+  HttpServer.bind().then((server) {
+    server.listen(
+        (HttpRequest request) {
+          var response = request.response;
+          Expect.isNull(response.headers.value('content-length'));
+          Expect.equals(-1, response.contentLength);
+          response.headers.set("content-length", 3);
+          Expect.equals("3", response.headers.value('content-length'));
+          Expect.equals(3, response.contentLength);
+          response.addString("xxx");
+          response.close();
+        });
+
+    var client = new HttpClient();
+    client.get("127.0.0.1", server.port, "/")
+        .then((request) => request.close())
+        .then((response) {
+          response.listen(
+              (_) { },
+              onDone: () {
+                client.close();
+                server.close();
+              });
+        });
+  });
+}
+
 void main() {
   testNoBody(5, false);
   testNoBody(5, true);
   testBody(5, false);
   testBody(5, true);
   testBodyChunked(5, false);
   testBodyChunked(5, true);
   testHttp10();
+  testSetContentLength();
 }