Update Spreadsheet.php

An "ambiguity" reported by Version that stopped being ambiguous with Php 7.0. I am very close to dropping this check.

Author: oleibman

src/PhpSpreadsheet/Spreadsheet.php

@@ -1125,7 +1125,7 @@ public function __clone()
                     $this->$key = [];
                     foreach ($currentCollection as $item) {
                         $clone = clone $item;
-                        $this->$key[] = $clone;
+                        $this->{$key}[] = $clone;
                     }
 
                     break;