tostring operator doesn't compile if argument is non const

Hello,
the example in the docs to bind the tostring operator 
(http://www.rasterbar.com/products/luabind/docs.html#operators) doesn't 
compile. I write it here for reference:

     #include <luabind/luabind.hpp>
     #include <luabind/operator.hpp>
     #include <ostream>
     class number {};
     std::ostream& operator<<(std::ostream& o, number&){ return o; }
     int main(){
         lua_State* L = NULL;
         using namespace luabind;
module(L)[class_<number>("number").def(tostring(self))];
     }

It compiles only if you put use this signature for <<:

std::ostream& operator<<(std::ostream& out, const A&)

Also, I would expect that const_self is required in the def, but it 
seems that it doesn't matter. Is this intended behaviour or a bug?

Cheers.

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