A bunch of improvements for conditional HTTP processing.

Fixed some typos in the code (fixed #10586). Added more tests. Made the
tests compatible with Python 2.3. Improved the documentation by putting
the good news and common use-case right up front.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10134 bcc190cf-cafb-0310-a4f2-bffc1f526a37