rails 如何清除clear session参照：http://stackoverflow.com/questions/684457/how-do-you-clear-a-single
rails 如何清除clear session
参照：http://stackoverflow.com/questions/684457/how-do-you-clear-a-single-entry-from-a-ruby-on-rails-session
http://stackoverflow.com/questions/2405635/how-to-empty-destroy-a-session-in-rails
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清除session中的单个对象:
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reset_session?