Node.js: ServerResponse.flushHeaders() implemented.

This closes #1006 issue on GitHub.

Reviewed-by: Andrew Clayton <a.clayton@nginx.com>

test/node/flush_headers/app.js

@@ -0,0 +1,7 @@
+
+require('http').createServer(function (req, res) {
+    res.setHeader('X-Header', 'blah');
+    res.flushHeaders();
+    res.flushHeaders(); // Should be idempotent.
+    res.end();
+}).listen(8080);

test/test_node_application.py

@@ -303,6 +303,10 @@ def test_node_application_get_header_names():
         'x-header',
     ], 'get header names'
 
+def test_node_application_flush_headers():
+    client.load('flush_headers')
+
+    assert client.get()['headers']['X-Header'] == 'blah'
 
 def test_node_application_has_header():
     client.load('has_header')