Add tests

tests/lib/util/extend.js

@@ -10,5 +10,8 @@ tape("test extend method", function(t) {
   t.notSame(obj, extended, "extended object doesn't have the same values as original object")
   t.notSame(obj.two, extended.two, "extended object value overwrites value from original object")
 
+  extended = extend(null)
+  t.equals(extended, null, "passing null returns null")
+
   t.end()
 })