Fix: prevented the binding illegal temporary to optional<const int&>

Older MSVC versions add illegal temporary when you want to assign from const integral value.

test/optional_test_ref_convert_assign_mutable_int.cpp

@@ -23,7 +23,8 @@
 
 int main()
 {
-  test_converting_assignment<int>();
+  test_converting_assignment<int, int>();
+  test_converting_assignment<int, const int>();
   
   return boost::report_errors();
 }