Allow CURLFile

While trying to use the `createImage()` method with PHP 5.5.9, I kept getting the error **upload requires a multipart/form-data POST with a valid filestream** when using the doc's examples in trying to upload a thumbnail image.

When I made my own cURL request, I discovered the issue is that newer PHP does not just consider the "old school" method of prefixing the '@' to the full path of a file to be "deprecated", it would appear it is no longer supported. I was only able to successfully upload images after I used the [CURLFile class](http://us2.php.net/manual/en/class.curlfile.php), which is mentioned in the CURLOPT_POSTFIELDS section of [their cURL doc](http://us1.php.net/manual/en/function.curl-setopt.php).

This little change simply makes sure string concatenation only happens on a string, since PHP 5.5 and up users will want this to be a CURLFile object instead.