[1.2.X] Fixed #14440 - Converted mail doctests to unittests.

Thanks to Rob Hudson for the patch and also to andialbrecht who filed a
similar patch that I didn't use.

Backport of r14143 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14144 bcc190cf-cafb-0310-a4f2-bffc1f526a37