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 = unicode(element.tag)
        if inner:
            g_inner = inner.encode('utf-8') if isinstance(inner, unicode) else inner
        else:
            g_inner = [] if element.text is None else [self.render_text(element.text, element, qwebcontext)]
            for current_node in element.iterchildren(tag=etree.Element):
                try:
                    g_inner.append(self.render_node(current_node, qwebcontext,
                        generated_attributes= name == "t" and generated_attributes or ''))
                except QWebException:
                    raise
                except Exception:
                    template = qwebcontext.get('__template__')
                    raise_qweb_exception(message="Could not render element %r" % element.tag, node=element, template=template)
        inner = "".join(g_inner)
        trim = template_attributes.get("trim", 0)
        if trim == 0:
            pass
        elif trim == 'left':
            inner = inner.lstrip()
        elif trim == 'right':
            inner = inner.rstrip()