Added safety check to invalidation middleware

in case the header key was not saved for some reason, it will return a blank list.
v0.1.1

* Added safety check for invalidation middleware, so that it will pass on project tests, where sometimes the caching isn't running as it would on a normal request-response.
* Added recommendation to use `django-response-timeout` for setting expires headers
Merge branch 'master' of github.com:saulshanabrook/django-dumper