title: confusion with base64 class methods usage
body: <p>Could anyone help me here to understand when we need to consider the
  below 4 methods:</p>
  
  <pre><code>strict_decode64(str)
  strict_encode64(bin)
  urlsafe_encode64(bin)
  urlsafe_decode64(str)
  </code></pre>
  
  <p>From the doc also I didn't get any examples. So examples with
  explanation might be helpful for me to understand.</p>
  
  <p>Thanks in advance</p>