fix: cache contract for memcached backend

set() should return a boolean. Currently, it always returns None. This
one-line fix resolves this issue.

Change-Id: I9d2827891dc8ee8904f3031ea75e5bef0ce30488
Fixes wrong hrefs when getting set of messages

Based on current implement, an additional path /messages will be
added, see http://git.openstack.org/cgit/openstack/marconi/queues
/transport/wsgi/messages.py#L208
However, the /messages path has been already included in the URL.
So this fix just remove it and use req.path directly.

Fixes bug 1240897

Change-Id: Ie4bf6d3df62a5b861a8fe8c6392eb8bd6bf57c2c
Merge "fix: cache contract for memcached backend"