One of the spec files sets `nil` value for env varialbe
`action_dispatch.request.formats`. Rather than setting `nil` value,
we can delete the `action_dispatch.request.formats` key from `env`
(which should be closer to real use case anyway). The reason is that
in Rails 5, `formats` method does not set default array of formats
if the env key is already set (even if set to `nil`).