Payload marker gets added on empty payload.

Even with an empty payload the Payload Marker (Payload Separator, End-of-Options Marker) would get added to the packet.

This confused the CoAP test server at http://coap.me/ which was used for the ETSI CoAP plugtest.

Draft-18 says: 
>If present and of non-zero length, it is prefixed by a fixed, one-byte Payload Marker (0xFF) which indicates the end of options and the start of the payload.

Also trimmed some trailing whitespace.