title: What is difference between ENV.fetch with block or with second param?
body: <p>I'm not sure whether:</p>
  
  <pre><code>ENV.fetch("RAILS_MAX_THREADS") { 5 }
  </code></pre>
  
  <p>and: </p>
  
  <pre><code>ENV.fetch("RAILS_MAX_THREADS", 5)
  </code></pre>
  
  <p>are the same or not. What is the difference?</p>