[1.2.X] Fixed #11877 -- Documented that HttpRequest.get_host() fails behind...

[1.2.X] Fixed #11877 -- Documented that HttpRequest.get_host() fails behind multiple reverse proxies, and added an example middleware solution. Thanks to Tom Evans for the report, and arnav for the patch.

Backport of [14493] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14494 bcc190cf-cafb-0310-a4f2-bffc1f526a37