Formatting problem with model::d2::point_xy<unsigned char>

This snippet
````
using namespace boost::geometry;
model::polygon> poly{{{0, 0}, {0, 5}, {5, 0}, {0, 0}}};
std::cout << dsv(poly);
````
will produce a bunch of funny characters instead of numerical values:
````
((( , ), ( , ♣), (♣, ), ( , )))
````