Impacted versions: 9.0, 9.0 enterprise

Steps to reproduce:
On current runbot duplicate any posted journal entry which is balanced

Current behavior:
Error message "Cannot create unbalanced entry" is shown

Expected behavior:
New Journal entry is created in state 'draft'

Video/Screenshot link (optional):
![selection_268](https://cloud.githubusercontent.com/assets/14974044/15007437/41f45a56-11d9-11e6-84f3-b297befaa521.png)

addons/account/models/account_move.py
move = super(AccountMove, self.with_context(check_move_validity=False, partner_id=vals.get('partner_id'))).create(vals)
        move.assert_balanced()
        return move
    @api.multi
    def write(self, vals):
        if 'line_ids' in vals:
            res = super(AccountMove, self.with_context(check_move_validity=False)).write(vals)
            self.assert_balanced()
        else:
            res = super(AccountMove, self).write(vals)
        return res
    @api.multi
    def post(self):