Clean up to use session methods and not _SESSION directly

Add session_unset for unsetAll and rename this method to "clear"

4dev/tests/Create/CoreLibsCreateSessionTest.php

@@ -444,7 +444,7 @@ final class CoreLibsCreateSessionTest extends TestCase
 			);
 		}
 		// unset all
-		$session->unsetAll();
+		$session->clear();
 		// check unset
 		foreach (array_keys($test_values) as $name) {
 			$this->assertEquals(