dgram: partially revert 18d457b

Revert "dgram: call send callback asynchronously" partially, since the
fix is now done in libuv.

Refs: https://github.com/nodejs/io.js/pull/1313
Refs: https://github.com/libuv/libuv/pull/371
PR-URL: https://github.com/nodejs/io.js/pull/1889
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/dgram.js

@@ -332,10 +332,7 @@ function afterSend(err) {
   if (err) {
     err = exceptionWithHostPort(err, 'send', this.address, this.port);
   }
-  var self = this;
-  setImmediate(function() {
-    self.callback(err, self.length);
-  });
+  this.callback(err, this.length);
 }