sql: DATE_TRUNC('week', <value>) truncates to midnight Sunday rather than Monday

[rafiss]

DATE_TRUNC('week', <value>) truncates to midnight Sunday on cockroachdb but midnight Monday on PostgreSQL.

This was discovered while testing with Django: cockroachdb/django-cockroachdb#92

PostgreSQL:

> SELECT DATE_TRUNC('week', '2019-11-13'::timestamp);
"2019-11-11 00:00:00"

cockroach:

> SELECT DATE_TRUNC('week', '2019-11-13'::timestamptz);
         date_trunc          
+---------------------------+
  2019-11-10 00:00:00+00:00