I always send uppercase commands:
this.send("PASS " + oAuth);
this.send("NICK " + login);
And have not had a problem, that said, I am not aware of Twitch having an issue with uppercase PASS or NICK commands, and not sure why you would have an issue.
Also, just using a straight telnet connection:
[Nyx] /opt/iobot/scripts/games
% telnet irc.chat.twitch.tv 6667
Trying 52.32.237.213…
Connected to irc-production-388322493.us-west-2.elb.amazonaws.com.
Escape character is ‘^]’.
PASS oauth:hiding_oauth_key
NICK illusionarybot
:tmi.twitch.tv 001 illusionarybot :Welcome, GLHF!
:tmi.twitch.tv 002 illusionarybot :Your host is tmi.twitch.tv
:tmi.twitch.tv 003 illusionarybot :This server is rather new
:tmi.twitch.tv 004 illusionarybot :-
:tmi.twitch.tv 375 illusionarybot :-
:tmi.twitch.tv 372 illusionarybot :You are in a maze of twisty passages, all alike.
:tmi.twitch.tv 376 illusionarybot :>