Fix for missing CURLOPT_HTTPHEADER initialization

When sending POST after last fix that changed: 
$curl_opts[CURLOPT_HTTPHEADER] = $headers;
to:
$curl_opts[CURLOPT_HTTPHEADER] = array_merge($headers,$curl_opts[CURLOPT_HTTPHEADER]);
it ends with:
"Notice: Undefined offset: 10023 in (here path)/vendor/lib/Pest.php line 112"
because CURLOPT_HTTPHEADER is not initialized with empty array.