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
t_render = None
        template_attributes = {}
        debugger = element.get('t-debug')
        if debugger is not None:
            if openerp.tools.config['dev_mode']:
                __import__(debugger).set_trace()  
            else:
                _logger.warning("@t-debug in template '%s' is only available in --dev mode" % qwebcontext['__template__'])
        for (attribute_name, attribute_value) in element.attrib.iteritems():
            attribute_name = unicode(attribute_name)
            if attribute_name == "groups":
                cr = qwebcontext.get('request') and qwebcontext['request'].cr or None