refactoring: Improve monad methods (iluwatar#2300)

Author: VxDxK

monad/src/main/java/com/iluwatar/monad/Validator.java

@@ -78,7 +78,7 @@ public static <T> Validator<T> of(T t) {
    * @param message    error message when object is invalid
    * @return this
    */
-  public Validator<T> validate(Predicate<T> validation, String message) {
+  public Validator<T> validate(Predicate<? super T> validation, String message) {
     if (!validation.test(obj)) {
       exceptions.add(new IllegalStateException(message));
     }
@@ -97,8 +97,8 @@ public Validator<T> validate(Predicate<T> validation, String message) {
    * @return this
    */
   public <U> Validator<T> validate(
-      Function<T, U> projection,
-      Predicate<U> validation,
+      Function<? super T, ? extends U> projection,
+      Predicate<? super U> validation,
       String message
   ) {
     return validate(projection.andThen(validation::test)::apply, message);