Update AssetFilter to use response methods.

Instead of directly outputting to the server, using the response object
will make AssetFilter more compatible with applications that straddling
PSR7 and dispatcher stacks.

Refs #9260

src/Routing/Filter/AssetFilter.php

@@ -87,9 +87,7 @@ class AssetFilter extends DispatcherFilter
 
         $pathSegments = explode('.', $url);
         $ext = array_pop($pathSegments);
-        $this->_deliverAsset($request, $response, $assetFile, $ext);
-
-        return $response;
+        return $this->_deliverAsset($request, $response, $assetFile, $ext);
     }
 
     /**
@@ -142,10 +140,7 @@ class AssetFilter extends DispatcherFilter
             $response->header('Content-Length', filesize($assetFile));
         }
         $response->cache(filemtime($assetFile), $this->_cacheTime);
-        $response->sendHeaders();
-        readfile($assetFile);
-        if ($compressionEnabled) {
-            ob_end_flush();
-        }
+        $response->file($assetFile);
+        return $response;
     }
 }

tests/TestCase/Routing/Filter/AssetFilterTest.php

@@ -245,14 +245,12 @@ class AssetFilterTest extends TestCase
         $request = new Request($url);
         $event = new Event('Dispatcher.beforeDispatch', $this, compact('request', 'response'));
 
-        ob_start();
-        $filter->beforeDispatch($event);
-        $result = ob_get_contents();
-        ob_end_clean();
+        $response = $filter->beforeDispatch($event);
+        $result = $response->getFile();
 
         $path = TEST_APP . str_replace('/', DS, $file);
         $file = file_get_contents($path);
-        $this->assertEquals($file, $result);
+        $this->assertEquals($file, $result->read());
 
         $expected = filesize($path);
         $headers = $response->header();