Do not yield stream, the AsyncServer API does not expect any value

...but it might, see xp-framework/networking#28

src/main/php/web/Response.class.php

@@ -216,7 +216,7 @@ public function transmit($source, $mediaType= 'application/octet-stream', $size=
     $out= $this->stream($size);
     try {
       while ($in->available()) {
-        yield 'write' => $out;
+        yield 'write' => null;
         $out->write($in->read());
       }
     } finally {