Remove several uses of the deprecated dict.has_key in favor of the in...

Remove several uses of the deprecated dict.has_key in favor of the in operator.  This is faster, has less code, and is in every single way better.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14392 bcc190cf-cafb-0310-a4f2-bffc1f526a37