Append `Vary: Cookies` header to response when session is accessed

To prevent caching proxies like Varnish from providing multiple users with the same cache, you can set a `Vary: Cookie` header. 

This pull request appends the header to each response when the session is accessed.

I came across this issue while working on Warehouse, PyPi 2.0. @dstufft wanted something that added the header to the response anytime the session was being used.

&mdash; @citruspi + @abadger