Set the port
This port is set to the string, which must contain only digits or be empty. An empty port string is distinct from having no port.
assert( url( "http://www.example.com" ).set_port( "8080" ).authority().buffer() == "www.example.com:8080" );
this->has_port() == true && this->port_number() == n && this->port() == std::to_string(n)
Strong guarantee. Calls to allocate may throw. Exceptions thrown on invalid input.
port = *DIGIT