Fix to write "DO" to respond to "IAC WILL x".

[junaruga]

When running a code analysis tool, I got an error

copy_paste_error: "DONT" in "IAC + DONT" looks like a copy-paste error.

for the below line. There are same lines both WILL[0] == $1[0] and WONT[0] == $1[0] cases in preprocess method.

self.write(IAC + DONT + $1[1..1])

I am not so confident for my modification, because I am not familiar with the telnet protocol.

[znz]

I think DONT means it cannot handle the option.
So I think it is intentional.

http://pcmicro.com/netfoss/telnet.html

WILL | DONT | Receiver says it cannot support the option. Option is not in effect.

[junaruga]

@znz alright. Thank you for checking it!