title: Yii2 compare email without case sensitive
body: <p>I use the simple compare validation rule offered by Yii2 like this: </p>
  
  <pre><code>[confirm_email', 'compare', 'compareAttribute'=&gt;'email', 'message'=&gt;"Emails don't match"],
  </code></pre>
  
  <p>The problem is that this rule compares two emails 100% including Case Sensitive which means <strong>email@test.com</strong> and <strong>email@Test.com</strong> will generate validation error. </p>
  
  <p>Is there a way to remove this Case Sensitive comparison from this rule? </p>