Fixed #3912 - Added simple type checking to template.Context.update() to help...

Fixed #3912 - Added simple type checking to template.Context.update() to help debugging broken context processors.  Thanks Jeremy Dunck


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8181 bcc190cf-cafb-0310-a4f2-bffc1f526a37