When updating contribution with tax having multiple price item, each item get the total tax amount

A bit similar to https://issues.civicrm.org/jira/browse/CRM-20259 but it can be reproduced from the UI without any API call.

Steps to reproduce:

1. Enable Invoice and Tax
2. Create a new financial account for taxes and add it to one financial type
3. Create a price set of type Contribution with multiple taxable price option (using the financial type above)
4. Create a new contribution from the backend and choose the new price set : choose several options -> save (the total and taxes seems ok)
5. Edit this contribution without changing anything -> save
6. All the taxes on the line items are equal to the contribution tax amount total (and i guess the financial tables are potentially wrong the same way)