Cron resource accepts a symbol as `weekday`
Resource::Cron#weekday accepts symbol arg.
Show a short message for invalid value for weekday.
CHEF-4848: Add additional unit tests
Enabled using special strings in cron. CHEF-2816
Documentation updates for contributions

* CHEF-2816 - Allow special strings to be used in cron (@reboot etc)
* CHEF-4848 - Cron resource accepts a symbol as `weekday`