title: Lodash isEqualWith with customizer not checking fields
body: <p>I'm trying to do an equality comparison ignoring undefined values using lodash.</p>
  
  <p>I'd expect the following to work:</p>
  
  <pre><code>console.log(
    isEqualWith(request.body, actualRequest.body.json, (a, b) =&gt; {
      console.log(a, b, a === undefined &amp;&amp; b === undefined ? true : undefined
      );
      return a === undefined &amp;&amp; b === undefined ? true : undefined;
    })
  );
  </code></pre>
  
  <p>However, the console output fails on the 'first' comparison (of the whole objects), even though the customizer returns undefined. Here's the output:</p>
  
  <pre><code>{ item1: undefined, item2: 'test'} { item2: 'test' } undefined
  false
  </code></pre>
  
  <p>It's never comparing anything except the first object. I'd expect an output like:</p>
  
  <pre><code>{ item1: undefined, item2: 'test'} { item2: 'test' } undefined
  undefined undefined true
  'test' 'test' undefined
  true
  </code></pre>
  
  <p>As, given the customizer returned undefined for the first check, then it would have gone through the fields of the object and performed the customzer check on those.</p>