Impacted versions: ODOO 10

Problem.
If i print a invoice with a negative amount, the pdf shows a vertical bar instead of a minus symbol.
![image](https://cloud.githubusercontent.com/assets/28800797/26237253/b07fadb0-3c74-11e7-86c2-c8688d82b5a5.png)

I have tried to install a new pdf reader, but with no result.
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):