Contribution forms with "Other amount" is not taxed properly

On a contribution page using an "other amount" field, CiviCRM does not seem to calculate the taxes correctly.

How to reproduce:

• enable invoicing and taxes
• create a 15% tax, add it to the "donations" financial type
• create a contribution page with a few options, as well as an "other amount" field.

When creating a new contribution (online, or using the backend), CiviCRM will not calculate the taxes correctly. It might use the first option of the price options, for example.

More details here:
http://civicrm.stackexchange.com/questions/4305/contribution-page-with-other-amounts-enabled-taxes-wrong

The issue was reproduced on the demo site.

Using a "real" priceset is a possible workaround for this issue.