title: Set product tax in Magento observer
body: <p>I'm using <code>sales_quote_add_item</code> and I'd like to change the tax class of a product.
  In observer, I retrieve product:</p>
  
  <pre><code>$event = $observer-&gt;getEvent();
  $item = $event-&gt;getQuoteItem();
  $product = $item-&gt;getProduct();
  </code></pre>
  
  <p>I tried  <code>$item-&gt;setTaxClassId()</code> and <code>$product-&gt;setTaxClassId()</code> methods but this didn't work.</p>
  
  <p>Any other approaches available?</p>