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)


_model = 'test_converter.monetary'
    def setUp(self):
        super(TestCurrencyExport, self).setUp()
        self.Currency = self.registry('res.currency')
        self.base = self.create(self.Currency, name="Source", symbol=u'source')
    def create(self, model, context=None, **values):
        return model.browse(
            self.cr, self.uid,
            model.create(self.cr, self.uid, values, context=context),
            context=context)
    def convert(self, obj, dest):
        converter = self.registry('ir.qweb.field.monetary')
        options = {
            'widget': 'monetary',
            'display_currency': 'c2'
        }
        context = dict(inherit_branding=True)
        converted = converter.to_html(
            self.cr, self.uid, 'value', obj, options,
            etree.Element('span'),
            {'field': 'obj.value', 'field-options': json.dumps(options)},
            '', ir_qweb.QWebContext(self.cr, self.uid, {'obj': obj, 'c2': dest, }),
            context=context,
        )
        return converted
    def test_currency_post(self):