Trying to use `NamedRoutes` with `servant-auth-server` currently results
in hideous error messages such as:
```
app/Main.hs:50:7: error:
• No instance for (Servant.Auth.Server.Internal.AddSetCookie.AddSetCookies
('Servant.Auth.Server.Internal.AddSetCookie.S
('Servant.Auth.Server.Internal.AddSetCookie.S
'Servant.Auth.Server.Internal.AddSetCookie.Z))
(AdminRoutes (Servant.Server.Internal.AsServerT Handler))
(ServerT
(Servant.Auth.Server.Internal.AddSetCookie.AddSetCookieApi
(Servant.Auth.Server.Internal.AddSetCookie.AddSetCookieApi
(NamedRoutes AdminRoutes)))
Handler))
arising from a use of 'serveWithContext'
• In the expression: serveWithContext (Proxy @API) ctx RootAPI {..}
```
This is because we didn't teach it how to recurse along `NamedRoutes`
trees and sprinkle headers at the tip of each branch.
This commit adds a test case and fixes the issue. In the process, it
also implements `ThrowAll` for `NamedRoutes`, which was necessary for
the test to run, and should also prove convenient for users.
