Prevent deprecation notices for implode, json_encode and substr modifiers

* Prevent deprecation notices for implode, json_encode and substr modifiers.
* unit tests

libs/plugins/modifiercompiler.json_encode.php

@@ -0,0 +1,11 @@
+<?php
+
+/**
+ * Smarty plugin
+ *
+ * @package    Smarty
+ * @subpackage PluginsModifierCompiler
+ */
+function smarty_modifiercompiler_json_encode($params) {
+	return 'json_encode(' . $params[0] . (isset($params[1]) ? ', (int) ' . $params[1] : '') . ')';
+}