title: How should i check if gravatar exist?
body: <p>using this gem 
  <a href="https://github.com/sinisterchipmunk/gravatar" rel="noreferrer">https://github.com/sinisterchipmunk/gravatar</a>
  how do i check if gravatar for specified email exist or not? I think i am missing some force default option? Because this</p>
  
  <pre><code>url = Gravatar.new("generic@example.com").image_url
  </code></pre>
  
  <p>always return a picture</p>