Bug in CaseInsensitiveMap

/**
 *
 * @author AlexLumpov
 */
public class CaseInsensitiveMapTest extends Assert {

	@Test
	public void testRetainAllKeys() {

		Map<String, String> map = new CaseInsensitiveMap<String>();
		map.put("1", "1");
		map.put("2", "2");
		map.put("3", "3");

		Collection<String> keysToRetain = Arrays.asList("3", "4", "5");
		HashSet<String> expected = new HashSet<String>(Arrays.asList("3"));

		boolean modified = map.keySet().retainAll(keysToRetain);

		assertEquals(true, modified);
		assertEquals(expected, map.keySet());
	}
}

java.lang.AssertionError: expected:<[3]> but was:<[2, 3]>