empty_message_copy: copy the rating as well (Jörg Knobloch discovered the bug)

Thanks Jörg.

src/message_cache.cc

@@ -268,6 +268,8 @@ namespace pEp {
             dst->opt_fields->next = dup(src->opt_fields);
         }
 
+        dst->rating = src->rating;
+
         return dst;
     }