gcoap_forward_proxy: copy Max-Age from forwarded Valid if it exists
gcoap_forward_proxy: copy Max-Age from forwarded Valid if it exists

kaspar030/RIOT,ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,ant9000/RIOT