in_tail: Fix error on setting page for binary file

How to reproduce:

  * Create a binary file by the following command:

        "ruby -e 'puts "\u3042"' > /tmp/xxx"

  * Open /daemon/setting/in_tail
  * Select "/tmp/xxx"
  * Error page is rendered
  * log/production.log says:

        ActionView::Template::Error (undefined method `join' for nil:NilClass):
            12:     = f.text_field :path, class: "form-control", disabled: true
            13:   = render partial: "shared/vue/in_tail_format", locals: { file: f.object.path, formats: @setting.known_formats, initialSelected: f.object.format || @setting.guess_format }
            14:
            15:   %pre= file_tail(@setting.path).join("\n")
            16:
            17:   %p
            18:     = f.submit t('terms.next'), class: "btn btn-lg btn-primary pull-right"
          app/views/fluentd/settings/in_tail/after_file_choose.html.haml:15:in `block in _app_views_fluentd_settings_in_tail_after_file_choose_html_haml___1713159989942914940_40548520'
Merge pull request #71 from kou/in-tail-fix-error-for-binary-file

in_tail: Fix error on setting page for binary file