Validation error in repr-digest examples

[martinthomson]

I can't see why this would be a problem, so this might need a little bit of digging.

draft-ietf-httpbis-digest-headers.xml:1531 'repr-digest: sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:': Binary Sequence failed to decode
draft-ietf-httpbis-digest-headers.xml:1561 'repr-digest: sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:': Binary Sequence failed to decode
draft-ietf-httpbis-digest-headers.xml:1803 'repr-digest: sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:': Binary Sequence failed to decode
draft-ietf-httpbis-digest-headers: http-lint ... FAIL