Add missing return types to offsetSet & offsetUnset (#29)

Co-authored-by: joel-44 <[email protected]>

Author: joel-loycom

src/Orders/Order.php

@@ -201,7 +201,7 @@ public function toArray(): array
      * @param  mixed  $value
      * @return void
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         if (is_null($offset)) {
             $this->purchase_units[] = $value;
@@ -216,7 +216,7 @@ public function offsetSet($offset, $value)
      * @param  mixed  $offset
      * @return void
      */
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         unset($this->purchase_units[$offset]);
     }