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_integer(self):
        converter = self.get_converter('integer')
        value = converter(42)
        self.assertEqual(value, "42")
class TestFloatExport(TestBasicExport):
    def setUp(self):
        super(TestFloatExport, self).setUp()
        self.registry('res.lang').write(self.cr, self.uid, [1], {
            'grouping': '[3,0]'
        })
    def test_float(self):
        converter = self.get_converter('float')
        value = converter(-42.0)
        self.assertEqual(value, u"\u201142.0")
        value = converter(42.0100)
        self.assertEqual(value, "42.01")
        value = converter(42.01234)
        self.assertEqual(value, "42.01234")
        value = converter(1234567.89)
        self.assertEqual(value, '1,234,567.89')
    def test_numeric(self):
        converter = self.get_converter('numeric')
        value = converter(42.0)
        self.assertEqual(value, '42.00')
        value = converter(42.01234)
        self.assertEqual(value, '42.01')
class TestCurrencyExport(TestExport):