gcoap_fileserver: don't abort on duplicate packet
gcoap_fileserver: don't abort on duplicate packet

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