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)


def test_image(self):
        field = self.get_field('binary')
        converter = self.registry('ir.qweb.field.image')
        with open(os.path.join(directory, 'test_vectors', 'image'), 'rb') as f:
            content = f.read()
        encoded_content = content.encode('base64')
        value = e(converter.value_to_html(
            self.cr, self.uid, encoded_content, field))
        self.assertEqual(
            value, '<img src="data:image/jpeg;base64,%s">' % (
                encoded_content
            ))
        with open(os.path.join(directory, 'test_vectors', 'pdf'), 'rb') as f:
            content = f.read()
        with self.assertRaises(ValueError):
            e(converter.value_to_html(
                self.cr, self.uid, 'binary', content.encode('base64'), field))
        with open(os.path.join(directory, 'test_vectors', 'pptx'), 'rb') as f:
            content = f.read()
        with self.assertRaises(ValueError):
            e(converter.value_to_html(
                self.cr, self.uid, 'binary', content.encode('base64'), field))
class TestSelectionExport(TestBasicExport):