[FIX] Fixes incorrect merge 3e1d5be951fb8
[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.
gdgellatly/OCB1,OpenPymeMx/OCB,gdgellatly/OCB1,grap/OCB,grap/OCB,vrenaville/ngo-addons-backport,grap/OCB,grap/OCB,camptocamp/ngo-addons-backport,gdgellatly/OCB1,OpenPymeMx/OCB,OpenPymeMx/OCB,gdgellatly/OCB1,camptocamp/ngo-addons-backport,vrenaville/ngo-addons-backport,vrenaville/ngo-addons-backport,vrenaville/ngo-addons-backport,vrenaville/ngo-addons-backport,camptocamp/ngo-addons-backport,camptocamp/ngo-addons-backport,OpenPymeMx/OCB,camptocamp/ngo-addons-backport,camptocamp/ngo-addons-backport,camptocamp/ngo-addons-backport,grap/OCB,OpenPymeMx/OCB,vrenaville/ngo-addons-backport,gdgellatly/OCB1,camptocamp/ngo-addons-backport,vrenaville/ngo-addons-backport,grap/OCB,gdgellatly/OCB1,grap/OCB,gdgellatly/OCB1,OpenPymeMx/OCB,OpenPymeMx/OCB,gdgellatly/OCB1,grap/OCB,OpenPymeMx/OCB,vrenaville/ngo-addons-backport