Fix for bug #4999
Seperated billdate generation from billnr

It's possible to set billnr in shopadmin before invoice generation, in
this case no billdate was generated
changed unit test myorderTest

sets a billdate and checks if payuntil date is 7 days from billdate
Merge branch 'myorderfix' of http://github.com/leofonic/oxideshop_ce into leofonic-myorderfix
Add "due date" back to delivery note, partial rollback of f8db23f
Fix for bug #4999 in exportDeliveryNote