Fix return value in ob_start callback argument

Author: ajgarlag

Factory/DiactorosFactory.php

@@ -133,7 +133,7 @@ public function createResponse(Response $symfonyResponse)
                 ob_start(function ($buffer) use ($stream) {
                     $stream->write($buffer);
 
-                    return false;
+                    return '';
                 });
 
                 $symfonyResponse->sendContent();

Factory/PsrHttpFactory.php

@@ -143,7 +143,7 @@ public function createResponse(Response $symfonyResponse)
                 ob_start(function ($buffer) use ($stream) {
                     $stream->write($buffer);
 
-                    return false;
+                    return '';
                 });
 
                 $symfonyResponse->sendContent();