Revert "close #2: ACK improperly includes Token"

This reverts commit 7f6fa83138e68e4db20dadc02f6d173658d9f165.

The fix is incorrect in the case where a non-Empty ACK is being generated,
as coap_add_token only works when it is invoked before options or data
are added.
close #2: ACK improperly includes Token

Use a cruder fix that assumes nobody else added options or data to an empty
message.
close #3: async example creates short packet

Calculate the size properly: 4 bytes payload, 1 byte payload marker, and
however long the token is.  Also check the return value from the data add.