Fixed #15785 -- Stopped HttpRequest.read() from reading beyond the end of a...

Fixed #15785 -- Stopped HttpRequest.read() from reading beyond the end of a wsgi.input stream and removed some redundant code in the multipartparser. Thanks, tomchristie, grahamd and isagalaev.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16479 bcc190cf-cafb-0310-a4f2-bffc1f526a37