Default header callbacks.

[ben-crowhurst]

Some default headers require dynamic processing. For example setting the 'Date' header field on each response.

settings->set_default_header( "Date", [ ]( void ) -> string
{
    const time_t now = ( date == 0 ) ? time( 0 ) : date;
    struct tm components = { };
    gmtime_r( &now, &components );

     char datastamp[ 50 ] = { };
     strftime( datastamp, sizeof datastamp, "%a, %d %b %Y %H:%M:%S GMT", &components );
     return datastamp;
} );

[ben-crowhurst]

This would be required functionality on restbed::Resource also.