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
pass
class AssetNotFound(AssetError):
    pass
class AssetsBundle(object):
    rx_css_import = re.compile("(@import[^;{]+;?)", re.M)
    rx_preprocess_imports = re.compile("""(@import\s?['"]([^'"]+)['"](;?))""")
    rx_css_split = re.compile("\/\*\! ([a-f0-9-]+) \*\/")
    def __init__(self, xmlid, cr=None, uid=None, context=None, registry=None, max_css_rules=MAX_CSS_RULES):
        self.xmlid = xmlid
        self.cr = request.cr if cr is None else cr
        self.uid = request.uid if uid is None else uid
        self.context = request.context if context is None else context
        self.registry = request.registry if registry is None else registry
        self.max_css_rules = max_css_rules
        self.javascripts = []
        self.stylesheets = []
        self.css_errors = []
        self.remains = []
        self._checksum = None
        context = self.context.copy()
        context['inherit_branding'] = False
        context['inherit_branding_auto'] = False
        context['rendering_bundle'] = True
        self.html = self.registry['ir.ui.view'].render(self.cr, self.uid, xmlid, context=context)
        self.parse()
    def parse(self):