Fix NewRelic+Webrick memory leak

configinitializersz_thread_sweeper.rb

class ZThreadSweeper
  def initialize(app)
    @app = app
  end

  def call(env)
    pre_keys  = Thread.current.keys
    result = @app.call(env)
    post_keys = Thread.current.keys

    # nuke
    post_keys.each { |k| Thread.current[k] = nil unless pre_keys.include?(k) }

    result
  end
end

Rails.configuration.middleware.insert_before 0, ZThreadSweeper