Fixed #7655 -- Added two assertions to mail.py to help people debug a common...

Fixed #7655 -- Added two assertions to mail.py to help people debug a common problem (sending strings instead of lists/tuples for 'to' or 'bcc'). Thanks, guettli

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7864 bcc190cf-cafb-0310-a4f2-bffc1f526a37