Fixed bug that wrong tax rate is used on invoice printing

To reproduce the bug, you create a cart price rule which reduces the cost if your payment type is bank payment by 3%.
Then the tax rate is miscalculated at the item (it becomes 18% instead of 19%) and thus the invoice printing is wrong.
To avoid this, I take the tax rate which is set at the item (if set) and calculate it on the fly only if it is not set.
See screenshot for the invoice problem.
![invoice-wrong-tax-rate-with-cart-rule](https://f.cloud.github.com/assets/4135790/430743/9a73f616-ae75-11e2-8281-c04a61702493.jpg)