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)


""" ``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]