geometry: Formatting problem with model::d2::point_xy

This snippet

    using namespace boost::geometry;
    model::polygon<model::d2::point_xy<unsigned char>> poly{{{0, 0}, {0, 5}, {5, 0}, {0, 0}}};
    std::cout << dsv(poly);

will produce a bunch of funny characters instead of numerical values:

((( ,  ), ( , ♣), (♣,  ), ( ,  )))