Pressing W or S while riding boats overrides state of A and D

The bug

In 16w04a the key W moves the boat forward and the key S backwards. However when you press first A or / and D and then W or / and S and release it, the paddles still move but the boat does not.
Additionally pressing S while holding A and D moves you backwards instead of slower forwards (backwards momentum is not subtracted from forwards momentum).

How to reproduce

1. Enter a boat
2. Press A and D
3. Press W and release it