Tax is not re-calculated when Membership added and the Amount is changed from the default. Not using a Priceset

Tax is not re-calculated when Membership added and the Amount is changed from the default. Not using a Priceset:

1. Enable sales tax and invoicing for CiviCRM
2. Set up a Membership Type and enable minimum amount. Do not create a Priceset for the Membership
3. Add a Membership for a Contact
4. Change the Amount for the Membership, so it is not the default Amount
5. Save the Membership
6. Locate the Contribution for the Membership
7. Note the Amount for the Contribution.
8. Change the status from Pending to Completed
9. Save the Contribution.
10. Compare the Contribution Amount before and after the edit.
11. Amount has now been changed.

Problem appears to be that the Tax is not re-calculated at step 4 when the Amount is changed and so the incorrect Tax is saved. Which buggers things up big time.

This is a continuation of ------CRM-19966------

Agileware Reference SUP-188

Have verified this problem with CiviCRM 4.7.19