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)


inner = self.eval_str(template_attributes["raw"], qwebcontext)
        return self.render_element(element, template_attributes, generated_attributes, qwebcontext, inner)
    def render_tag_esc(self, element, template_attributes, generated_attributes, qwebcontext):
        options = json.loads(template_attributes.get('esc-options') or '{}')
        widget = self.get_widget_for(options.get('widget'))
        inner = widget.format(template_attributes['esc'], options, qwebcontext)
        return self.render_element(element, template_attributes, generated_attributes, qwebcontext, inner)
    def _iterate(self, iterable):
        if isinstance (iterable, collections.Mapping):
            return iterable.iteritems()
        return itertools.izip(*itertools.tee(iterable))
    def render_tag_foreach(self, element, template_attributes, generated_attributes, qwebcontext):
        expr = template_attributes["foreach"]
        enum = self.eval_object(expr, qwebcontext)
        if enum is None:
            template = qwebcontext.get('__template__')
            raise QWebException("foreach enumerator %r is not defined while rendering template %r" % (expr, template), template=template)
        if isinstance(enum, int):
            enum = range(enum)
        varname = template_attributes['as'].replace('.', '_')
        copy_qwebcontext = qwebcontext.copy()
        size = None
        if isinstance(enum, collections.Sized):