Fix wrong default rounding of quantity for invoice

The default digits of quantity is 2 so the default rounding must be 0.01

issue4374
review11671003
(grafted from 099ed666ac0c7fe944746d92c3c1cd1746c0fe8f)

--HG--
branch : 2.6
extra : source : 099ed666ac0c7fe944746d92c3c1cd1746c0fe8f