Voucher + Assign Role removes all other roles from user

Steps to reproduce:
1. As Administrator, manually assign role A to a Test User.
2. Use Registration Code Role Assignment module to add role B when they enter the correct code.
3. Enable and configure Registration Code Voucher System module.
4. Log in as Test User.
5. Go to My account and Edit. Enter the code from step 2.
6. Log back in as Administrator and observe that Test User now has role B, but has lost role A.

Simple one line patch attached.