wsgi.Middleware forward-compatibility with webob 1.2b1 or later

Response.request is None by default
(http://docs.webob.org/en/latest/news.html#b1), but is used in the
CacheFilter WSGI middleware.

Backport of https://review.openstack.org/#/c/17794/

Change-Id: I28f5ca92fe517f4f56af934799db32650e079ba7
Merge "wsgi.Middleware forward-compatibility with webob 1.2b1 or later" into stable/folsom