ZF-7510: Zend_Auth_Adapter_DbTable to use the default adapter

Description

It would be convenient to have Zend_Auth_Adapter_DbTable default to the default db adapter.

Comments

Assigning to Ralph

patch attached

updated patch

If no database adatper is provided into Zend_Auth_Adapter_DbTable constructor, it tries to fetch the default one (Zend_Db_Table_Abstract::getDefaultAdapter())