Fixed #7109 -- Quote certain values before passing them for substitution in

Field.db_type().

This fixes a problem with using reserved words for field names in Oracle. Only
affects Oracle at the moment, but the same changes could easily be used by
other backends if they are required (requires changing creation.py, only).

This commit also reverts [7501] so that if the fix doesn't work, it will show
up in the tests (and if it does work, the tests will prevent us from breaking
it again).


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7743 bcc190cf-cafb-0310-a4f2-bffc1f526a37