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
super(QWeb, self).__init__(pool, cr)
        self._render_tag = self.prefixed_methods('render_tag_')
        self._render_att = self.prefixed_methods('render_att_')
    def prefixed_methods(self, prefix):
        """ Extracts all methods prefixed by ``prefix``, and returns a mapping
        of (t-name, method) where the t-name is the method name with prefix
        removed and underscore converted to dashes
        :param str prefix:
        :return: dict
        """
        n_prefix = len(prefix)
        return dict(
            (name[n_prefix:].replace('_', '-'), getattr(type(self), name))
            for name in dir(self)
            if name.startswith(prefix)
        )
    def register_tag(self, tag, func):