bugfix: #24 Empty ACK improperly includes Token
bugfix: #24 Empty ACK improperly includes Token

Included the patch proposed by tbahls to fix this issue. Now, the
ACK message is always truncated to its bare header when the message code
is 0.00.

gebart/libcoap,thecodemaiden/libcoap-fork,RIOT-OS/libcoap,sgso/libcoap,authmillenon/libcoap,fredericgermain/libcoap,fredericgermain/libcoap,nmacs/libcoap,sgso/libcoap,fredericgermain/libcoap,authmillenon/libcoap,RIOT-OS/libcoap,authmillenon/libcoap,gebart/libcoap,sgso/libcoap,gebart/libcoap,PolymorhicCode/libcoap,thecodemaiden/libcoap-fork,nmacs/libcoap,nmacs/libcoap