restore pdu after sending
restore pdu after sending

this way, the send-then-delete approach to pdus keeps working (meaning
less lwip specific patches), and for retransmission it's required to
keep the pdu around anyway (until we switch the send queue to a pbuf
queue)

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