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)


converter = self.get_converter('date')
        value = converter('2011-05-03')
        # 
        self.assertEqual(value, '05/03/2011')
    def test_datetime(self):
        converter = self.get_converter('datetime')
        value = converter('2011-05-03 11:12:13')
        # 
        self.assertEqual(value, '05/03/2011 00:12:13')
    def test_custom_format(self):
        converter = self.get_converter('datetime')
        converter2 = self.get_converter('date')
        opts = {'format': 'MMMM d'}
        value = converter('2011-03-02 11:12:13', options=opts)
        value2 = converter2('2001-03-02', options=opts)
        self.assertEqual(
            value,
            'March 2'
        )
        self.assertEqual(
            value2,
            'March 2'
        )
class TestDurationExport(TestBasicExport):