Kernel#clone and Kernel#dup no longer raise TypeError; fix #4974

In ISO, those methods should raise `TypeError`, but the spec has been
changed.  The change was discussed in [Feature#12979].

src/kernel.c

@@ -325,7 +325,7 @@ mrb_obj_clone(mrb_state *mrb, mrb_value self)
   mrb_value clone;
 
   if (mrb_immediate_p(self)) {
-    mrb_raisef(mrb, E_TYPE_ERROR, "can't clone %v", self);
+    return self;
   }
   if (mrb_sclass_p(self)) {
     mrb_raise(mrb, E_TYPE_ERROR, "can't clone singleton class");
@@ -366,7 +366,7 @@ mrb_obj_dup(mrb_state *mrb, mrb_value obj)
   mrb_value dup;
 
   if (mrb_immediate_p(obj)) {
-    mrb_raisef(mrb, E_TYPE_ERROR, "can't dup %v", obj);
+    return obj;
   }
   if (mrb_sclass_p(obj)) {
     mrb_raise(mrb, E_TYPE_ERROR, "can't dup singleton class");

test/t/kernel.rb

@@ -197,17 +197,6 @@ assert('Kernel#dup', '15.3.1.3.9') do
   a.set(2)
   c = a.dup
 
-  immutables = [ 1, :foo, true, false, nil ]
-  error_count = 0
-  immutables.each do |i|
-    begin
-      i.dup
-    rescue TypeError
-      error_count += 1
-    end
-  end
-
-  assert_equal immutables.size, error_count
   assert_equal 2, a.get
   assert_equal 1, b.get
   assert_equal 2, c.get