Allow to include uninitialized modules [Bug #18177]

The module that is about to be included is considered initialized.

class.c

@@ -917,7 +917,8 @@ ensure_includable(VALUE klass, VALUE module)
     rb_class_modify_check(klass);
     Check_Type(module, T_MODULE);
     if (RMODULE_UNINITIALIZED(module)) {
-        rb_raise(rb_eArgError, "uninitialized module");
+        RB_OBJ_WRITE(module, &RCLASS(module)->super, 0);
+        /* no more re-initialization */
     }
     if (!NIL_P(rb_refinement_module_get_refined_class(module))) {
 	rb_raise(rb_eArgError, "refinement module is not allowed");

test/ruby/test_module.rb

@@ -432,6 +432,18 @@ def x
         initialize_copy(Module.new)
       end
     end
+
+    m = Class.new(Module) do
+      def initialize_copy(other)
+        # leave uninitialized
+      end
+    end.new.dup
+    c = Class.new
+    assert_operator(c.include(m), :<, m)
+    cp = Module.instance_method(:initialize_copy)
+    assert_raise(TypeError) do
+      cp.bind_call(m, Module.new)
+    end
   end
 
   def test_dup