Fixed a return type error. (#18)

Fixes an error when null is returned instead of a string.

Author: tobz-nz

src/Message/PurchaseResponse.php

@@ -63,7 +63,7 @@ public function getTransactionReference(): string
  */
  public function getMessage(): string
  {
- return $this->data['ErrorMessage'] ?? $this->data['Message'] ?? null;
+ return $this->data['ErrorMessage'] ?? $this->data['Message'] ?? 'Something went wrong';
  }
 
  /**