message.rb: correct @plainmessage and @message for CTCP messages

message.rb: correct @plainmessage and @message for CTCP messages

After CTCP parsing, @plainmessage was not defined correctly (as it still
contained the CTCP command), and @plainmessage and @message were not
being cleaned up correctly.

This manifested itself e.g. with spurious \001ACTION and other strange
unusual byte sequence popping up in markov-generated text.

Fix by redefining @plainmessage and @message appropriately in CTCP
handling.