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
""" This special 't-call' tag can be used in order to aggregate/minify javascript and css assets"""
        if len(element):
            
            
            
            template = qwebcontext.get('__template__')
            raise QWebException("t-call-assets cannot contain children nodes", template=template)
        xmlid = template_attributes['call-assets']
        cr, uid, context = [getattr(qwebcontext, attr) for attr in ('cr', 'uid', 'context')]
        bundle = AssetsBundle(xmlid, cr=cr, uid=uid, context=context, registry=self.pool)
        css = self.get_attr_bool(template_attributes.get('css'), default=True)
        js = self.get_attr_bool(template_attributes.get('js'), default=True)
        async = self.get_attr_bool(template_attributes.get('async'), default=False)
        return bundle.to_html(css=css, js=js, debug=bool(qwebcontext.get('debug')), async=async, qwebcontext=qwebcontext)
    def render_tag_set(self, element, template_attributes, generated_attributes, qwebcontext):