Backend Membership with Priceset applies taxes twice to line_item

Steps to reproduce:

• Enable Taxes, Add a tax account with a 10% tax
• Add the tax account to the Membership financial type
• Create a new priceset for memberships, with radio options for the main membership options.
• From a contact record, add a new membership for that contact, select an option from the priceset.

Notice how to total amount is OK, but when we view the contribution, the line item shows a unit_price with the tax_amount included, so then the total_line ends up adding the taxes again.

(setting this as "major" because the bug results in incorrect data)