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)


""" Precision should be the currency's, not the float field's
        """
        currency = self.create(self.Currency, name="Test", symbol=u"test",)
        obj = self.create(self.Model, value=0.1234567)
        converted = self.convert(obj, dest=currency)
        self.assertEqual(
            converted,
            '<span data-oe-model="{obj._model._name}" data-oe-id="{obj.id}" '
                  'data-oe-field="value" data-oe-type="monetary" '
                  'data-oe-expression="obj.value">'
                      '<span class="oe_currency_value">0.12</span>'
                      u'\N{NO-BREAK SPACE}{symbol}</span>'.format(
                obj=obj,
                symbol=currency.symbol.encode('utf-8')
            ).encode('utf-8'),)
class TestTextExport(TestBasicExport):
    def test_text(self):