next_run returns wrong value

[opussf]

Description: Given a cron string that fires once a week, and the date given to next_run is on the last day of a month with 31 days, the returned month value it not incremented.

Expected: Returned month value is incremented. Returns datetime( 2016, 9, 7, 20, 0 )

Actual: month value is same as the one given. Returns datetime( 2016, 8, 7, 20, 0 )

Reproduce:
Cron string "0 18 * * 3" (6pm every Wednesday).
next_run( datetime.datetime(2016, 8, 31, 20, 01) ).