Add check of 'SET DateStyle TO DEFAULT'.

src/test/regress/expected/timestamp.out

@@ -1578,3 +1578,4 @@ SELECT '' AS to_timestamp_8, to_timestamp('2000January09Sunday', 'YYYYFMMonthDDF
                 | Sun Jan 09 00:00:00 2000 PST
 (1 row)
 
+SET DateStyle TO DEFAULT;

src/test/regress/sql/timestamp.sql

@@ -222,3 +222,5 @@ SELECT '' AS to_timestamp_6, to_timestamp('15 "text bettween quote marks" 98 54
 SELECT '' AS to_timestamp_7, to_timestamp('05121445482000', 'MMDDHHMISSYYYY');    
 
 SELECT '' AS to_timestamp_8, to_timestamp('2000January09Sunday', 'YYYYFMMonthDDFMDay');
+
+SET DateStyle TO DEFAULT;