FIX: Pre Tax Total difference between edition mode and saved mode

There's a difference between edition mode and saved mode with Pre Tax Total value. It seems the Pre Tax Total is correct when in editing mode of the quote/invoice and changed to the Total with Tax value when quote/invoice is saved.

See https://discussions.vtiger.com/discussion/193665/pre-tax-total-difference-between-edition-mode-and-saved-mode-in-quote-invoice