To avoid an unfortunately common user-error, rename QuerySet.as_sql().

This was never a public API method, so this is backwards compatible, unless
you're poking at the internals. Refs #10352.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9928 bcc190cf-cafb-0310-a4f2-bffc1f526a37