[-] MO: Fix rounding error in new order email

The previous values are rounded to 2 decimals in the following method:
Order::getOrders =» Order::getProductsDetail =» Order::setProductPrices
This is due to backwards compatibility with 1.4, but it can lead to
rounding errors in the product lines in the email.
Instead, here we are using other Core provided values.

See example screenshot: http://cl.ly/image/0e0g2D1u0Y45
(the minus-minus on the coupon line is another issue)

mailalerts.php

@@ -281,7 +281,7 @@ public function hookActionValidateOrder($params)
 		Product::addCustomizationPrice($products, $customized_datas);
 		foreach ($products as $key => $product)
 		{
-			$unit_price = Product::getTaxCalculationMethod($customer->id) == PS_TAX_EXC ? $product['product_price'] : $product['product_price_wt'];
+			$unit_price = Product::getTaxCalculationMethod($customer->id) == PS_TAX_EXC ? $product['unit_price_tax_excl'] : $product['unit_price_tax_incl'];
 
 			$customization_text = '';
 			if (isset($customized_datas[$product['product_id']][$product['product_attribute_id']]))