Preventing PHP to segfault on PHP 7

PHP 7 issue #69329 causes the mime_content_type to fail in files smaller than 3 bytes. This change just increase the file on cake to avoid the error. The content of this file is not checked on the tests, just the mime content type.

tests/TestCase/View/Helper/RssHelperTest.php

@@ -598,7 +598,7 @@ class RssHelperTest extends TestCase
         $tmpFile = WWW_ROOT . 'tests/cakephp.file.test.tmp';
         $File = new File($tmpFile, true);
 
-        $this->assertTrue($File->write('123'), 'Could not write to ' . $tmpFile);
+        $this->assertTrue($File->write('1234'), 'Could not write to ' . $tmpFile);
 
         clearstatcache();