Fix halt(invoke) breaks its argument

When I passed an rack array instance variable to `halt` like

    halt(@forbidden)

the `halt` method breaks its argument, actually in `invoke` method. 

    def invoke
      res = catch(:halt) { yield }
      res = [res] if Fixnum === res or String === res
      if Array === res and Fixnum === res.first
        status(res.shift)
        body(res.pop)
        headers(*res)
      elsif res.respond_to? :each
        body res
      end
      nil # avoid double setting the same response tuple twice
    end

See there is `res.shift` and `res.pop`. It finally breaks my `@forbidden` instance variable which is returned by `catch(:halt) { yield }`. 
My question is "is this a desired behavior?" If not, let's fix it. This is a pull request to fix it.