Make it possible to pass functions to change output streaming

[thekid]

Can be used for implementing HTTP compression:

use web\Filter;
use web\io\Output;

class WriteCompressed extends Output {

  /* Implementation omitted for brevity */ 

}

class CompressResponses implements Filter {

  /* Negotiation omitted for brevity */

  public function filter($req, $res, $invocation) {
    if ($compression= $this->negotiate($req->header('Accept-Encoding'))) {
      $res->streaming(function($res) use($compression) {
        return new WriteCompressed($res, $compression);
      });
    }

    return $invocation->proceed($req, $res);
  }
}

[thekid]

This library is built on top of this PR: https://github.com/xp-forge/web-compression

[thekid]

This fails when nesting these calls, e.g. one filter being the developer console, and the other being compression, each of them overwriting the streaming implementation instead of nesting it.