Fixes to Akka HTTP server

I'm working my way through the Play integration tests and fixing up any problems when they're run with the experimental Akka HTTP server. The commits in this PR fix the `FlashCookieSpec` and the `AssetsSpec` integration tests (and probably some other tests too).

Note: the `FlashCookieSpec` will currently fail for one day each week due to a [bug](https://github.com/akka/akka/pull/16514) in Akka HTTP with parsing Sunday dates. Our CI will still work because we're not running our integration tests with the Akka HTTP server as part of our CI.