::utility::datetime cannot be used as a key

[PBRCW]

When trying to use ::utility::datetime as a key for an entity compilation fails:
.\Release\include\cpprest/asyncrt_utils.h(212): error C2679: binary '<<': no operator found which takes a right-hand operand of type 'const utility::datetime' (or there is no acceptable conversion)

This is due to class datetime failing to overload the << and >> stream operators. As the class already has two methods from_string() and to_string() this should be faily easy to fix.