title: Strange "pattern not match" error on fluentd
body: <p>Can someone tell if its normal that fluentd raise this error in <strong>td-agent.log</strong> file?</p>
  
  <pre><code>2015-07-31 13:15:19 +0000 [warn]: pattern not match: "- - - [31/Jul/2015:13:15:19 +0000] GET http://172.31.108.218/ HTTP/1.1 200 0 \"-\" \"ELB-HealthChecker/1.0\""
  </code></pre>
  
  <p>While this is a well formated apache2 log:</p>
  
  <pre><code>- - - [31/Jul/2015:13:15:19 +0000] GET http://172.31.108.218/ HTTP/1.1 200 0 \"-\" \"ELB-HealthChecker/1.0\"
  </code></pre>
  
  <p>And here is the source configuration:</p>
  
  <pre><code>&lt;source&gt;
    type tail
    format apache2
    path /var/log/varnish/varnishncsa.log
    pos_file /var/log/td-agent/tmp/access.log.pos
    tag "apache2.varnish-access"
  &lt;/source&gt;
  </code></pre>
  
  <p>I can't figure out what's wrong there above.</p>