Fix `Module#dup` to frozen module

Before this patch:

  $ bin/mruby -e 'p Module.new.freeze.dup.frozen?'  #=> true

After this patch (same as Ruby):

  $ bin/mruby -e 'p Module.new.freeze.dup.frozen?'  #=> false

2 files changed, 9 insertions, 1 deletions

diff --git a/src/class.c b/src/class.c
index 2961284f5..65d21ad4f 100644
--- a/src/class.c
+++ b/src/class.c
@@ -2078,6 +2078,14 @@ mrb_mod_eqq(mrb_state *mrb, mrb_value mod)
 }
 
 static mrb_value
+mrb_mod_dup(mrb_state *mrb, mrb_value self)
+{
+  mrb_value mod = mrb_obj_clone(mrb, self);
+  MRB_UNSET_FROZEN_FLAG(mrb_obj_ptr(mod));
+  return mod;
+}
+
+static mrb_value
 mrb_mod_module_function(mrb_state *mrb, mrb_value mod)
 {
   mrb_value *argv;
@@ -2207,6 +2215,7 @@ mrb_init_class(mrb_state *mrb)
   mrb_define_method(mrb, mod, "method_defined?",         mrb_mod_method_defined,   MRB_ARGS_REQ(1)); /* 15.2.2.4.34 */
   mrb_define_method(mrb, mod, "define_method",           mod_define_method,        MRB_ARGS_ARG(1,1));
   mrb_define_method(mrb, mod, "===",                     mrb_mod_eqq,              MRB_ARGS_REQ(1)); /* 15.2.2.4.7 */
+  mrb_define_method(mrb, mod, "dup",                     mrb_mod_dup,              MRB_ARGS_NONE());
 
   mrb_undef_method(mrb, cls, "append_features");
   mrb_undef_method(mrb, cls, "extend_object");
diff --git a/src/kernel.c b/src/kernel.c
index a3c2d2ec6..f223be9fc 100644
--- a/src/kernel.c
+++ b/src/kernel.c
@@ -805,5 +805,4 @@ mrb_init_kernel(mrb_state *mrb)
   mrb_define_method(mrb, krn, "__to_str",                   mrb_to_str,                      MRB_ARGS_NONE()); /* internal */
 
   mrb_include_module(mrb, mrb->object_class, mrb->kernel_module);
-  mrb_define_alias(mrb, mrb->module_class, "dup", "clone"); /* XXX */
 }