FinatraServer should take the generic Filters, not SimpleFilters

I am not too sure if there was ever a reason to use the limited subtype instead of Filter, but this was requiring us to wrap the [CorsFilter](https://github.com/twitter/finagle/blob/8c30bd2ddddff64e5d51634e8ef678c47dd67177/finagle-http/src/main/scala/com/twitter/finagle/http/filter/Cors.scala) into a dummy SimpleFilter.