self must be the newly created object in initialization block; close #1858

mrbgems/mruby-struct/src/struct.c

@@ -382,7 +382,7 @@ mrb_struct_s_def(mrb_state *mrb, mrb_value klass)
   }
   st = make_struct(mrb, name, rest, struct_class(mrb));
   if (!mrb_nil_p(b)) {
-    mrb_funcall(mrb, b, "call", 1, st);
+    mrb_yield_internal(mrb, b, 1, &st, st, mrb_class_ptr(klass));
   }
 
   return st;