[FIX] calendar: fix read method to avoid to read computed field start_date(time)

Read(['start', 'start_datetime', 'start_date']) make the cache corrupt
for computed field start_datetime or start_date.

Now we don't read (for nothing ?) these 2 fields.

We can remove the skip for the test #500fabf1

Before this commit, a simple :
x = self.create({'name':'subject', 'allday': False, 'start': foo, 'stop': bar})

create randomly an event withtout start_datetime