[1.1.X] Fixed #12234 -- Create additional indexes that use the appropriate...

[1.1.X] Fixed #12234 -- Create additional indexes that use the appropriate operation class for PostgreSQL `varchar` and `text` columns when `db_index=True`.

Backport of r11912 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@11913 bcc190cf-cafb-0310-a4f2-bffc1f526a37