Fix "Notice: Array to string conversion"

Fix "Notice: Array to string conversion" when $value in curl_setopt($ch, CURLOPT_POSTFIELDS, $value) is an array that contains an empty array. Example that causes Notice:

$data = array(
    'foo' => 'bar',
    'baz' => array(
    ),
);
Merge pull request #1 from user52/patch-1

Fix "Notice: Array to string conversion"