Invoke specialized write for <response>

The `<response>` type has a specialized version of `write` and `write-element` defined in `server/core/response.dylan` for `<response>` and `<byte-string>`, but `copy-to-end` here instead called a more generic version for `<stream>` and `<sequence>`.

This meant the `<response>` was being written to as a normal stream, instead of using the `maybe-send-chunk` method and writing to `response.response-stream`.

Coercing the parameters to the types expected by the `write` and `write-element` methods in `server/core/response.dylan` fixes an issue with `<directory-resource>` serving static files.