Tax rule is not applied with empty tax when creating purchases from purchase requests

See http://www.tryton.org/~irclog/latest.log.html#t2017-05-23%2016:02

assigned to @pokoli

added purchasing type::behavior + 1 deleted label

Correct irc log link: http://www.tryton.org/~irclog/2017-05-23.log.html#t2017-05-23%2016:02

added 1 deleted label and removed 1 deleted label

review35011002 updated at https://codereview.tryton.org/35011002/#ps1

New changeset 40924c151dc8 by Sergi Almacellas Abellana in branch 'default':
Apply tax rule with empty taxes when creating purchase
http://hg.tryton.org/modules/purchase_request/rev/40924c151dc8

added 1 deleted label and removed 1 deleted label

closed

New changeset bac87e5727dd by Sergi Almacellas Abellana in branch '4.4':
Apply tax rule with empty taxes when creating purchase
http://hg.tryton.org/modules/purchase_request/rev/bac87e5727dd

New changeset 285365181759 by Sergi Almacellas Abellana in branch '4.2':
Apply tax rule with empty taxes when creating purchase
http://hg.tryton.org/modules/purchase_request/rev/285365181759