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
* ``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,