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.