Bug 853990 Fix suds cache usage

We made a change to use our own cache with Suds, but it turns out that
Suds uses named type checking to validate the cache instead of duck
typing, so we need to inherit from Suds's cache even though we don't
use anything from it.
Merge pull request #77 from dpoirier/bug-853990-fix-suds-cache

Bug 853990 Fix suds cache usage