"@daily cmd" can't be re-scheduled

Hi,

First thanks for providing this excellent tool.

When using this python-crontab, there is one tricky bug. If a job is scheduled to run at 00:00, the crob job will becomes @daily cmd and this job's schedule can never be changed, at least the following code won't work

job.minute.on(minute)
job.hour.on(hour)

Thank you!