bffe1bd6 has introduced jsonpath .datetime() method, but default formats
for time and timestamp contain excess space between time and timezone.  This
commit removes this excess space making behavior of .datetime() method
standard-compliant.

Discussion: https://postgr.es/m/94321be0-cc96-1a81-b6df-796f437f7c66%40postgrespro.ru
Author: Nikita Glukhov
Backpatch-through: 13