Fix refund after vesting has passed #5
Description
From Sohkai's review: _refund() should check if the vesting's cliff date has been reached when calculating the amount to burn: because refund() can be called at any time, a user who waits until after the cliff period to be refunded may experience odd behaviour (either they receive some vested tokens or are unable to process the refund because the token manager fails to burn the expected amount of tokens)