add content-type only if valid
add content-type only if valid

nmacs/libcoap,nmacs/libcoap,joaopedrotaveira/libcoap,joaopedrotaveira/libcoap,authmillenon/libcoap,markushx/libcoap,authmillenon/libcoap,OlegHahm/libcoap,RIOT-OS/libcoap,authmillenon/libcoap,fredericgermain/libcoap,joaopedrotaveira/libcoap,pabigot/libcoap,sgso/libcoap,fredericgermain/libcoap,nmacs/libcoap,thecodemaiden/libcoap-fork,gebart/libcoap,pabigot/libcoap,gebart/libcoap,sgso/libcoap,alexaring/libcoap,OlegHahm/libcoap,markushx/libcoap,OlegHahm/libcoap,pabigot/libcoap,markushx/libcoap,alexaring/libcoap,RIOT-OS/libcoap,PolymorhicCode/libcoap,sgso/libcoap,alexaring/libcoap,thecodemaiden/libcoap-fork,fredericgermain/libcoap,gebart/libcoap