In some contexts the response from is not properly cacheable

In some IIS configurations, the cassette response is not properly cacheable.

Using ```response.Cache.SetSlidingExpiration(true);``` makes it work and it should not have any adverse effect.

Fix inspired by this [page](http://geekswithblogs.net/cicorias/archive/2011/03/14/differences-when-running-with-outputcache-managed-module-under-asp.net-iis7.x.aspx)