message.Writer.Write() adds missing \r's, but the buffer of "last bytes
written" was only being updated while writing the message headers, not while
writing the body. so for Write()'s in the body section (depending on
buffering), we were compensating based on the "last bytes written" as set
during the last write in the header section. that could cause a spurious \r to
be added when a Write starts with \n while the previous Write did properly
end with \r.
for issue #117, thanks haraldrudell for reporting and investigating
ecf60568b4