title: WP_Query not working when using tax_query
body: <p>I'm trying to do a <code>WP_Query</code> in a custom page template to retrieve posts that are in the category with id 4003 <strong>AND</strong> either in category with ID <code>$category_id</code> <strong>OR</strong> have the tag with slug <code>$category_slug</code>.<br>
  I'm using this code:  </p>
  
  <pre><code>&lt;?php $args3 = array(
      'post_type' =&gt; 'post',
      'posts_per_page' =&gt; 3,
      'tax_query' =&gt; array(
          'relation' =&gt; 'AND',
          array(
              'taxonomy' =&gt; 'category',
              'field' =&gt; 'term_id',
              'terms' =&gt; 4003,
          ),
          array(
              'relation' =&gt; 'OR',
              array(
                  'taxonomy' =&gt; 'tag',
                  'field' =&gt; 'slug',
                  'terms' =&gt; $category_slug
              ),
              array(
                  'taxonomy' =&gt; 'category',
                  'field' =&gt; 'term_id',
                  'terms' =&gt; $category_id
              )
          )
      ),
      );
      $query3 = new WP_Query( $args3 );
  ?&gt;
  </code></pre>
  
  <p>It seems that the problem is the <code>tag</code> query, because posts with both categories 4003 and <code>$category_id</code> are correctly retrieved. I <em>know</em> that there are posts with category 4003 and tag <code>$category_slug</code> but I cannot retrieve them.<br>
  What am I doing wrong?</p>