fix testlogger

@tagomoris

I got an error like

    "/Users/seo.naotoshi/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/logger.rb:370:in `<'",
    "/Users/seo.naotoshi/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/logger.rb:370:in `add'",
    "/Users/seo.naotoshi/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/logger.rb:456:in `error'",

when I tested my plugin (fluent-plugin-grep).