Fixed #13547 -- Made sure the ISO 8601 date formatting introduced in r12058...

Fixed #13547 -- Made sure the ISO 8601 date formatting introduced in r12058 uses "T" as the separator between the date and the time value to increase real world usefulness.

While the ISO standard permits the use of a space instead of "T" for readability, it does have an impact on standards like HTML5 which rely on specific rules made in RFC 3339.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13266 bcc190cf-cafb-0310-a4f2-bffc1f526a37