Changing namespace does not reset the internal cache version

Using the following code, "key1" from the "test_" namespace cannot be retrieved:

        $cache->setNamespace('test_');
        $cache->save('key1', 'test');
        $cache->contains('key1'); // TRUE

        $cache->setNamespace('test2_');
        $cache->save('key2', 'test');
        $cache->contains('key2'); // TRUE
		
        $cache->deleteAll();
		
        $cache->setNamespace('test_');
        $cache->contains('key1'); // FALSE - but this should be TRUE 


Fix and testcase in pull request.