[FIX] sale: multi-company tax filtering

When the admin creates a SO, he has access to all taxes (since ir.rules
do not apply to him); therefore in some cases, the SO could be created
with SO lines that have taxes from other companies. This usually does
not make sense.

From this revision on, only taxes that are in the same company as the
SO are applied to the lines. If no company is set, then no filtering
is done.