Fixed #17481. pbkdf2 hashes no longer ommit leading zeros.

Some existing user passwords may need to be reset or converted 
after this change. See the 1.4-beta release notes for more details.

Thanks bhuztez for the report and initial patch, claudep for the test.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@17418 bcc190cf-cafb-0310-a4f2-bffc1f526a37