@tekka Your rewrite actually fixed a problem I was having. For what ever reason I could not send messages from my ESP8266 based gateway to my test node. At the same time acknowledgement worked.
Log of the original lib:

0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=fail:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=fail:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=fail:1

Same sketch with just your commit added:

0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=ok:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=ok:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=ok:1

Nothing else was changed.
I didn't do any extensive testing. Just a quick check to see if your commit fixed my problem.