FIX HTTPBulkToolsResponse.addSuccessRecords() fn

This function was trying to merge 2 arrays with PHP fn `array_push()`
which was failing if the second parameter was array as well. This fix
replaces `array_push()` with `array_merge()` fn.

Resolves #187

src/BulkTools/HTTPBulkToolsResponse.php

@@ -197,7 +197,7 @@ public function addSuccessRecord($record)
      */
     public function addSuccessRecords(SS_List $records)
     {
-        array_push($this->successRecords, $records->toArray());
+        $this->successRecords = array_merge($this->successRecords, $records->toArray());
         return $this;
     }