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);