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
self._render_tag[tag] = func
    def get_template(self, name, qwebcontext):
        origin_template = qwebcontext.get('__caller__') or qwebcontext['__stack__'][0]
        try:
            document = qwebcontext.loader(name)
        except ValueError:
            raise_qweb_exception(QWebTemplateNotFound, message="Loader could not find template %r" % name, template=origin_template)
        if hasattr(document, 'documentElement'):
            dom = document
        elif document.startswith("<?xml"):
            dom = etree.fromstring(document)
        else:
            dom = etree.parse(document).getroot()
        res_id = isinstance(name, (int, long)) and name or None
        for node in dom:
            if node.get('t-name') or (res_id and node.tag == "t"):
                return node
        raise QWebTemplateNotFound("Template %r not found" % name, template=origin_template)
    def eval(self, expr, qwebcontext):