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/base/ir/ir_qweb.py
_name = 'ir.qweb.field.html'
    _inherit = 'ir.qweb.field'
    def value_to_html(self, cr, uid, value, field, options=None, context=None):
        return HTMLSafe(value or '')
class ImageConverter(osv.AbstractModel):
    """ ``image`` widget rendering, inserts a data:uri-using image tag in the
    document. May be overridden by e.g. the website module to generate links
    instead.
    .. todo:: what happens if different output need different converters? e.g.
              reports may need embedded images or FS links whereas website
              needs website-aware
    """
    _name = 'ir.qweb.field.image'
    _inherit = 'ir.qweb.field'
    def value_to_html(self, cr, uid, value, field, options=None, context=None):
        try:
            image = Image.open(cStringIO.StringIO(value.decode('base64')))
            image.verify()
        except IOError:
            raise ValueError("Non-image binary fields can not be converted to HTML")
        except: 
            raise ValueError("Invalid image content")
        return HTMLSafe('<img src="data:%s;base64,%s">' % (Image.MIME[image.format], value))
class MonetaryConverter(osv.AbstractModel):