[TW-1814] ISO8601 week format does not accept "day 7" and "day 0" is weird

_Ellington Santos on 2016-06-07T12:30:52Z says:_

$ t 138 mod due:2016-W23-7
'2016-W23-7' is not a valid date in the 'Y-WV-w' format.

But it accepts day 0 incorrectly:

$ t 138 mod due:2016-W23-0
Modifying task 138 'Tentar entrar nas gigs not prod e atualizar para 1.1.0'.
Modified 1 task.

$ t 138
[...]
Due 2016-06-05 00:00:00
[...]

$t undo

The last modification was made 6/7/2016

Prior Values Current Values
due 2016-W22-0T00:00