[1.2.X] Improved unicode-type, ASCII-convertible header handling in

HttpResponse.

Fixed #8765. Thanks to SmileyChris and semenov for working on this one.

Backport of r13740 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13748 bcc190cf-cafb-0310-a4f2-bffc1f526a37