Fix warnings

[eugeneius]

test/channel_test.rb:6: warning: assigned but unused variable - c1
test/channel_test.rb:6: warning: assigned but unused variable - c2
test/channel_test.rb:6: warning: assigned but unused variable - c3
lib/net/ssh/multi/pending_connection.rb:46: warning: `*' interpreted as argument prefix
lib/net/ssh/multi/session_actions.rb:124: warning: shadowing outer local variable - ch
lib/net/ssh/multi/session_actions.rb:134: warning: shadowing outer local variable - ch
lib/net/ssh/multi/session_actions.rb:144: warning: shadowing outer local variable - ch
test/session_actions_test.rb:97: warning: shadowing outer local variable - c
lib/net/ssh/multi/server.rb:138: warning: instance variable @session not initialized