Withdrawal credential misses its prefix

According to specification the first byte of the withdrawal credential should be a prefix (see https://github.com/ethereum/eth2.0-specs/blob/dev/specs/phase0/deposit-contract.md#withdrawal-credentials) where here 
https://github.com/CarlBeek/eth2.0-deposit-tooling/blob/master/deposit.py#L95 it's the plain hash without prefix.