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 test_many2one(self):
        Sub = self.registry('test_converter.test_model.sub')
        id0 = self.Model.create(self.cr, self.uid, {
            'many2one': Sub.create(self.cr, self.uid, {'name': "Foo"})
        })
        id1 = self.Model.create(self.cr, self.uid, {
            'many2one': Sub.create(self.cr, self.uid, {'name': "Fo<b>o</b>"})
        })
        def converter(record):
            model = self.registry('ir.qweb.field.many2one')
            return e(model.record_to_html(self.cr, self.uid, 'many2one', record))
        value = converter(self.Model.browse(self.cr, self.uid, id0))
        self.assertEqual(value, "Foo")
        value = converter(self.Model.browse(self.cr, self.uid, id1))
        self.assertEqual(value, "Fo&lt;b&gt;o&lt;/b&gt;")
class TestBinaryExport(TestBasicExport):