Added 'Setting headers' and 'Telling the browser to treat the response as a...

Added 'Setting headers' and 'Telling the browser to treat the response as a file attachment' sections to docs/request_response.txt

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7510 bcc190cf-cafb-0310-a4f2-bffc1f526a37