keepalive for on mesh commissioner sent to leader aloc. (ARMmbed#1821)

Author: deepakvenugopal

source/6LoWPAN/Thread/thread_commissioning_api.c

@@ -903,6 +903,7 @@ int thread_commissioning_petition_keep_alive(int8_t interface_id, commissioning_
     } else {
         uri_ptr = THREAD_URI_LEADER_KEEP_ALIVE;
         service_id = this->coap_service_id;
+        thread_management_get_leader_aloc(this->interface_id, this->destination_address);
     }
 
     ptr = payload;