[1.2.X] Fixed #14513 -- check fields with underscores for validity when...

[1.2.X] Fixed #14513 -- check fields with underscores for validity when ordering. Bonus points to Klaas van Schelven.

Backport from trunk (r14315)

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14317 bcc190cf-cafb-0310-a4f2-bffc1f526a37