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.
* ``readonly``, has this attribute if the field is readonly
        * ``expression``, the original expression
        :returns: iterable of (attribute name, attribute value) pairs.
        """
        field = record._fields[field_name]
        field_type = get_field_type(field, options)
        data = [
            ('data-oe-model', record._name),
            ('data-oe-id', record.id),
            ('data-oe-field', field_name),
            ('data-oe-type', field_type),
            ('data-oe-expression', t_att['field']),
        ]
        if field.readonly:
            data.append(('data-oe-readonly', 1))
        return data
    def value_to_html(self, cr, uid, value, field, options=None, context=None):
        """ value_to_html(cr, uid, value, field, options=None, context=None)
        Converts a single value to its HTML version/output
        """
        if not value: return ''
        return value
    def record_to_html(self, cr, uid, field_name, record, options=None, context=None):
        """ record_to_html(cr, uid, field_name, record, options=None, context=None)
        Converts the specified field of the browse_record ``record`` to HTML
        """
        field = record._fields[field_name]
        return self.value_to_html(
            cr, uid, record[field_name], field, options=options, context=context)
    def to_html(self, cr, uid, field_name, record, options,