fix testlogger

@tagomoris

I got an error as bolows when I tested my plugin (fluent-plugin-grep). 

    "/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'",