[FIX] Fixes incorrect merge 3e1d5be951fb8

Merge conflict at account/account_invoice.py (commit ab797f0cd8)
was incorrectly resolved. As a result, cannot validate invoices with 
tax line.
Merge pull request #82 from alhashash/ocb-7.0-fix_conflict_3e1d5be951

[FIX] Fix incorrect merge 3e1d5be951fb8