title: wp_insert_post with custom taxonomy
body: <p>I registered a custom taxonomy with this code:</p>
  
  <pre><code>register_taxonomy( 'user_r_category', array( 'user_r' ), $args );
  </code></pre>
  
  <p>Now I try to insert a post to 'user_r_category' taxonomy within the category ID 7 and the post_type 'user_r':</p>
  
  <pre><code>$new_post = array(
            //'ID' =&gt; '',
            'post_author' =&gt; $current_user-&gt;ID, 
            //'post_category' =&gt; array(7),
            'post_type'   =&gt; 'user_r',
            'post_content' =&gt; $r_textarea, 
            'post_title' =&gt; $r_title,
            'tax_input'    =&gt; array(
                              'user_r_category' =&gt; array( 7 )
                          ),
            'post_status' =&gt; 'publish'
          );
  
      $post_id = wp_insert_post($new_post);
  </code></pre>
  
  <p>The post was created, but not with the category 7. How could this work?</p>