Fixed #5917 -- More error robustness in date parsing in SelectDateWidget, plus

keep the original date selected on redisplay, even if it was bogus (e.g. 31
Feb). Patch from SmileyChris.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7337 bcc190cf-cafb-0310-a4f2-bffc1f526a37