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.
openerp/addons/base/ir/ir_qweb.py
""" ``monetary`` converter, has a mandatory option
    ``display_currency`` only if field is not of type Monetary.
    Otherwise, if we are in presence of a monetary field, the field definition must
    have a currency_field attribute set.
    The currency is used for formatting *and rounding* of the float value. It
    is assumed that the linked res_currency has a non-empty rounding value and
    res.currency's ``round`` method is used to perform rounding.
    .. note:: the monetary converter internally adds the qweb context to its
              options mapping, so that the context is available to callees.
              It's set under the ``_qweb_context`` key.
    """
    _name = 'ir.qweb.field.monetary'
    _inherit = 'ir.qweb.field'
    def to_html(self, cr, uid, field_name, record, options,
                source_element, t_att, g_att, qweb_context, context=None):
        options['_qweb_context'] = qweb_context
        return super(MonetaryConverter, self).to_html(
            cr, uid, field_name, record, options,
            source_element, t_att, g_att, qweb_context, context=context)
    def record_to_html(self, cr, uid, field_name, record, options, context=None):
        if context is None:
            context = {}
        Currency = self.pool['res.currency']
        cur_field = record._fields[field_name]
        display_currency = False
        
        if cur_field.type == 'monetary' and cur_field.currency_field:
            display_currency = record[cur_field.currency_field]