Fixed 14513 -- check fields with underscores for validity when ordering. Bonus...

Fixed 14513 -- check fields with underscores for validity when ordering. Bonus points to Klaas van Schelven.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14315 bcc190cf-cafb-0310-a4f2-bffc1f526a37