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

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

Author: thekid

src/main/php/web/handler/FilesFrom.class.php

@@ -77,7 +77,7 @@ private function copy($out, $file, $range) {
 
     $length= $range->length();
     while ($length && $chunk= $file->read(min(self::CHUNKSIZE, $length))) {
-      yield 'write' => $out;
+      yield 'write' => null;
       $out->write($chunk);
       $length-= strlen($chunk);
     }
@@ -129,7 +129,7 @@ public function serve($request, $response, $target, $mimeType= null) {
         $file->open(File::READ);
         try {
           do {
-            yield 'write' => $out;
+            yield 'write' => null;
             $out->write($file->read(self::CHUNKSIZE));
           } while (!$file->eof());
         } finally {