issue#17 Changing line_item quantity and unit price doesn't affect tax amount on both Add and Edit form

[monishdeb]

Steps to replicate:

1. Enable Sale Tax and create/modify Financial Type that has tax rate. Let say Donation FT has 10% tax rate.
2. Create a contribution choosing a price-field that is configured with Donation FT or simply choose Donation FT and do a backoffice contribution.
3. Go to the edit contribution and change the line-item quantity and/or unit price via Edit form

BEFORE

AFTER

[Edzelopez]

Works as expected.