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.qweb'
    _inherit = 'ir.qweb.field.many2one'
    def record_to_html(self, cr, uid, field_name, record, options=None, context=None):
        if not getattr(record, field_name):
            return None
        view = getattr(record, field_name)
        if view._model._name != "ir.ui.view":
            _logger.warning("%s.%s must be a 'ir.ui.view' model." % (record, field_name))
            return None
        ctx = (context or {}).copy()
        ctx['object'] = record
        html = view.render(ctx, engine='ir.qweb', context=ctx).decode('utf8')
        return HTMLSafe(html)
class QwebWidget(osv.AbstractModel):
    _name = 'ir.qweb.widget'
    def _format(self, inner, options, qwebcontext):
        return self.pool['ir.qweb'].eval_str(inner, qwebcontext)
    def format(self, inner, options, qwebcontext):
        return escape(self._format(inner, options, qwebcontext))
class QwebWidgetMonetary(osv.AbstractModel):