Update MailchimpRequest.php

use strict comparison

Author: andrii-pukhalevych

src/Requests/MailchimpRequest.php

@@ -456,7 +456,7 @@ public function reset()
             }
 
             $this->$key = null;
-            if ($key == 'headers') {
+            if ('headers' === $key) {
                 $this->$key = [];
             }
         }