title: Ignited Datatables selecting all columns even when column names are specified
body: <p>I can't seem to get this working and it's driving me nuts.
  I'm using CodeIgniter's Ignited Datatables library I found on <a href="https://github.com/IgnitedDatatables/Ignited-Datatables" rel="nofollow noreferrer">github</a></p>
  
  <p>I have the below method for getting records from a table joined to another:</p>
  
  <pre><code>public function get_templates_ajax() {
          $this-&gt;datatables-&gt;select('tem.template_id, tem.name, tem.vat, COUNT(itm.template_id) AS total_items');
          $this-&gt;datatables-&gt;from('templates tem');
          $this-&gt;datatables-&gt;join('items itm', 'tem.template_id = itm.template_id', 'left');
          $this-&gt;datatables-&gt;where('tem.user_id', $this-&gt;session-&gt;user_id);
          $this-&gt;datatables-&gt;group_by('tem.template_id');
          $this-&gt;datatables-&gt;add_column('actions', '&lt;a href="javascript:void(0);" class="edit_record btn btn-info" data-id="$1" data-name="$2" data-vat="$3"&gt;Edit&lt;/a&gt;&lt;a href="javascript:void(0);" class="delete_record btn btn-danger" data-id="$1"&gt;Delete&lt;/a&gt;', 'tem.template_id, tem.name, tem.vat');
          return $this-&gt;datatables-&gt;generate();
  } 
  </code></pre>
  
  <p>Yes, I have a field named <code>user_id</code> in both tables, but I'm not selecting the column, however, I get this error: Duplicate column name 'user_id'.</p>
  
  <p>I specified the columns I want selected as seen below:</p>
  
  <pre><code>$this-&gt;datatables-&gt;select('tem.template_id, tem.name, tem.vat, COUNT(itm.template_id) AS total_items');
  </code></pre>
  
  <p>I know if I wanted to select the user_id column, I'd use an alias, but I'm not selecting it.</p>
  
  <p>Doing <code>$this-&gt;db-&gt;last_query();</code> dumped this SQL:</p>
  
  <pre><code>SELECT COUNT(*) FROM (SELECT * FROM `templates` `tem` LEFT JOIN `items` `itm` ON `tem`.`template_id` = `itm`.`template_id` WHERE `tem`.`user_id` = '1' GROUP BY `tem`.`template_id`) SqueryAux
  </code></pre>
  
  <p>So it seems that the select method is being ignored while it is selecting all columns from both tables.
  What could I be doing wrong?</p>