[1.1.X] Fixed #12390 -- `Distance` and `Area` objects now support...

[1.1.X] Fixed #12390 -- `Distance` and `Area` objects now support multiplication when they are the right-hand side.

Backport of r11898 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@11899 bcc190cf-cafb-0310-a4f2-bffc1f526a37