title: Return :checked checkboxes value with is()
body: <p>I have sets of checkboxes so I assigned each set in a variable.</p>
  
  <pre><code>var $set1 = $('.suppliers'), $set2 = $('.products'), $set3 = $('.customer')//etc...
  </code></pre>
  
  <p>Then somewhere on my code, I want to put each set :checked values in an array:</p>
  
  <pre><code>var set1Arr = [];
  
  $set1.is(':checked').each(function() {
     set1Arr.push($(this).val());
  });
  </code></pre>
  
  <p>But nothing is returned, I also debugged and on console it says Object has no method 'each'. 
  I tried using not(':checked') and it is working okay, it returned the unchecked values.</p>