Simple curl change

Summary: This is stemming from issue #4002, but I don't think fixes it.

With code like this:

```
<?php

$file = new CurlFile(__DIR__. '/curl_testdata1.txt');
$ch = curl_init();
curl_setopt($ch, CURLOPT_POSTFIELDS, array("file" => $file));
curl_exec($ch);
var_dump($file->name);
var_dump($file->postname);
```

`$postname` is empty in both PHP 5.6 and HHVM.

It will be tested by the issue raiser though on a real request when it lands to GitHub.

At the very least, this is a consistency fix. At the very best, a miracle happens

The claim is that on requests, HHVM is setting filename empty like this:

```
Content-Disposition: form-data; name="file"; filename=""
Content-Type: application/octet-stream
```

while PHP 5.6 it is:

```
Content-Disposition: form-data; name="file"; filename="/home/vagrant/Sites/poshonpennies.com/app/routes.php"
Content-Type: application/octet-stream
```

Reviewed By: @ptarjan

Differential Revision: D1621800
Merge pull request #4058 from JoelMarcey/HHVM-3.3-curl_fix_cherry_pick

Simple curl change