Impacted versions: Odoo 11 on windows server 2012

Steps to reproduce: 
Create a new out invoice, select a product and set a negative amount for unit price.
Print the invoice.

Current behavior:
in PDF the char "-" is shown as "|" even if the total amount is correct (amount1 - amount2 = total)

Expected behavior:
the printed invoice should show the char "-"

Video/Screenshot link (optional):
![image](https://user-images.githubusercontent.com/37513316/42877323-8c712b2e-8a89-11e8-9690-27f00e08b6b2.png)

this is the negative item on odoo:
![image](https://user-images.githubusercontent.com/37513316/42877389-bf04eba2-8a89-11e8-91c0-e38059673b42.png)


import json
import os
import datetime
from lxml import etree
from openerp.tests import common
from openerp.tools import html_escape as e
from openerp.addons.base.ir import ir_qweb
directory = os.path.dirname(__file__)
class TestExport(common.TransactionCase):
    _model = None
    def setUp(self):
        super(TestExport, self).setUp()
        self.Model = self.registry(self._model)
    def get_field(self, name):
        return self.Model._fields[name]
    def get_converter(self, name, type=None):
        field = self.get_field(name)
        for postfix in type, field.type, '':
            fs = ['ir', 'qweb', 'field']
            if postfix is None: continue
            if postfix: fs.append(postfix)
            try:
                model = self.registry('.'.join(fs))
                break
            except KeyError: pass
        return lambda value, options=None, context=None: e(model.value_to_html(
            self.cr, self.uid, value, field, options=options, context=context))
class TestBasicExport(TestExport):