fix missing tax amount from invoice

Given 

you place an order with a payment method using the payment action
Mage_Payment_Model_Method_Abstract::ACTION_AUTHORIZE_CAPTURE

and the payment method supports capturing

protected $_canCapture = true;

and

the quote contains an item which has been discounted 100% (or at least so much that the tax amount is zero)

and 

the quote item has hidden taxes applied to it

Then the hidden taxes are missing from the invoice. This result is that the invoice total is too low by the hidden tax amount.

This patch fixes that.