Description:
------------
Trying to set multidimensional array as CURLOPT_POSTFIELDS issues a NOTICE, and I suppose that end user should make a string out (that way it's allright).

However, I see no way to pass a multidimensional array and a file together using CURLFile - which is encouraged as of 5.5.0 (and since '@' notation is deprecated).

Test script:
---------------
<?php

$data =
[
    'checked' => ['a', 'b', 'c'],
];

$ch = curl_init();
curl_setopt ($ch, CURLOPT_POSTFIELDS, http_build_query ($data));
// works fine

curl_setopt ($ch, CURLOPT_POSTFIELDS, $data);
// Notice: Array to string conversion in test.php on line 12

$data = 
[
    'checked' => ['a', 'b', 'c'],
    'file'    => new CURLFile ('test.php')
];
curl_setopt ($ch, CURLOPT_POSTFIELDS, $data);
// Notice: Array to string conversion in test.php on line 20