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)


openerp/addons/test_converter/tests/test_html.py
def setUp(self):
        super(TestDurationExport, self).setUp()
        # 
        self.registry('res.lang').load_lang(self.cr, self.uid, 'fr_FR')
    def test_negative(self):
        converter = self.get_converter('float', 'duration')
        with self.assertRaises(ValueError):
            converter(-4)
    def test_missing_unit(self):
        converter = self.get_converter('float', 'duration')
        with self.assertRaises(ValueError):
            converter(4)
    def test_basic(self):
        converter = self.get_converter('float', 'duration')
        result = converter(4, {'unit': 'hour'}, {'lang': 'fr_FR'})
        self.assertEqual(result, u'4 heures')
        result = converter(50, {'unit': 'second'}, {'lang': 'fr_FR'})
        self.assertEqual(result, u'50 secondes')
    def test_multiple(self):
        converter = self.get_converter('float', 'duration')
        result = converter(1.5, {'unit': 'hour'}, {'lang': 'fr_FR'})
        self.assertEqual(result, u"1 heure 30 minutes")
        result = converter(72, {'unit': 'second'}, {'lang': 'fr_FR'})
        self.assertEqual(result, u"1 minute 12 secondes")
class TestRelativeDatetime(TestBasicExport):