There is a delegator to get failure app, introduced in 4629bee and tuned
in 24b26026. The latter commit introduced a bit of logic, however, no
tests are included into commit. Needless to say this resulted in a
broken code.
The point is that `env["warden.options"][:scope]` returns a string.
However, `Devise.mappings` is a hash with symbol keys.
Adding tests and converting scope to symbol here.
