change the method of instantiateClass from default to private and add test cases

Author: wei.zw

src/test/java/org/apache/ibatis/reflection/factory/DefaultObjectFactoryTest.java

@@ -21,7 +21,6 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
-import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;