Old HTML ampersand strict compatibility

- FIX: e107::url() now puts "&" in the query string instead of "&" for compliance with the older,
       looser definition of ambiguous ampersands in the HTML specification.
       Fixes: #4054
- FIX: Typo in comment
- FIX: Clear the core/e107/addons/e_url registry (cache) because if a plugin is installed after that
       cache is initialized, the cache is not updated anymore. The plugin's e_url is therefore not
       loaded, so SEF URLs won't be generated for that plugin until the cache is regenerated.
- NEW: Test for #4054
- FIX: e_pluginTest::testGetFields() expects the initial condition of the "forum" plugin to be
       uninstalled.

e107_handlers/plugin_class.php

@@ -140,6 +140,7 @@ class e_plugin
 	{
 		$this->_installed = array();
 		$this->_addons = array();
+		e107::setRegistry('core/e107/addons/e_url');
 
 		$this->_init(true);
 		$this->_initIDs();