Fix protocol breaking when user tries to send empty chunk
Fix protocol breaking when user tries to send empty chunk

The {data, IsFin, Data} uses IsFin to indicate whether this
is the last chunk, while chunked transfer-encoding uses the
length of Data, and ends when it is 0. We must therefore not
send chunks with empty data.

ninenines/cowboy,hairyhum/cowboy,bsmr-erlang/cowboy,rabbitmq/cowboy,kivra/cowboy,turtleDeng/cowboy,K2InformaticsGmbH/cowboy,CrankWheel/cowboy