title: Calculation is not working with custom Cell Rendering with AJAX in Handsontable
body: <p>In my table(Handsontable) I have four columns <strong>Cars</strong>, <strong>A</strong>, <strong>B</strong> and <strong>C</strong>.
  Data for <strong>Cars</strong> and <strong>A</strong> columns are loaded from MySQL database. (like <strong><a href="https://github.com/handsontable/handsontable/blob/master/demo/php.html" rel="nofollow noreferrer">PHP Demo</a></strong>).</p>
  
  <p>Data of Column <strong>B</strong> is populated from MySQL database via AJAX depending on the value of <strong>Cars</strong>. The code is as follows:</p>
  
  <pre><code>{type: { renderer : function (instance, TD, row, col, prop, value, cellProperties) {
                  Handsontable.TextCell.renderer.apply(this, arguments);
                  var cr,prc;
                  cr = instance.getDataAtCell(row, 0);
                  prc = instance.getDataAtCell(row, 1);
                  $.ajax({
                      url: "php/act-go-t.php",
                      data: {cars: cr, price: prc}, 
                      dataType: 'json',
                      type: 'POST',
                      success: function (res) {
                          if (res.result[0].tax === null) {
                              TD.innerHTML = '0.000';
                              }
                              else {
                              TD.innerHTML = res.result[0].tax;
                              }
                          },
                          error: function () {
                          TD.innerHTML = '0.000';
                          }
                      });                 
                  }}}
  </code></pre>
  
  <p>The C column is the <strong>SUM</strong> of <strong>A</strong> and <strong>B</strong> and the code is:</p>
  
  <pre><code>{type : { renderer : function (instance, TD, row, col, prop, value, cellProperties) {
                      Handsontable.TextCell.renderer.apply(this, arguments);
                      var a,b;
                      a = instance.getDataAtCell(row, 1);
                      b = instance.getDataAtCell(row, 2);                     
                      TD.innerHTML = +a + +b;
                  }}}
  </code></pre>
  
  <p>The problem is although the value is loaded in <strong>B</strong> but the addition is not working. If I set the type of <strong>B</strong> column to numeric(<code>{type: numeric}</code>) except the AJAX, the addition is working fine.</p>
  
  <p>Result with AJAX:</p>
  
  <blockquote>
  <pre><code>+----------+------+-------+--------------+
  | Cars     |   A  |    B  |            C | 
  +----------+------+-------+--------------+
  | Nissan   |   20 |    10 |          20  |  
  | Honda    |    5 |     6 |           5  |    
  +----------+------+-------+--------------+
  </code></pre>
  </blockquote>
  
  <p>Result without AJAX:</p>
  
  <blockquote>
  <pre><code>+----------+------+-------+-------------+
  | Cars     |   A  |    B  |           C |
  +----------+------+-------+-------------+
  | Nissan   |   20 |    10 |         30  |  
  | Honda    |    5 |     6 |          11 |    -
  +----------+------+-------+-------------+
  </code></pre>
  </blockquote>
  
  <p>Can anybody please tell me if I am missing something?</p>