(PUP-3728) Fix off by one error in cron month names

When a name, such as "January", is used to specify a month in the Cron type,
the resulting numeric value would be off by one (i.e. "0"). This is because
month is unique among "namable" crotab entries in that it does not use 0-based
indexing. This patch updates the spec tests to enforce this expectation and
fixes the issue by padding out the list of acceptable month names so that
indices start at 1.