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
source_element, t_att, g_att, qweb_context, context=None):
        """ to_html(cr, uid, field_name, record, options, source_element, t_att, g_att, qweb_context, context=None)
        Converts a ``t-field`` to its HTML output. A ``t-field`` may be
        extended by a ``t-field-options``, which is a JSON-serialized mapping
        of configuration values.
        A default configuration key is ``widget`` which can override the
        field's own ``_type``.
        """
        try:
            content = self.record_to_html(cr, uid, field_name, record, options, context=context)
            if options.get('html-escape', True):
                content = escape(content)
            elif hasattr(content, '__html__'):
                content = content.__html__()
        except Exception:
            _logger.warning("Could not get field %s for model %s",
                            field_name, record._name, exc_info=True)
            content = None
        inherit_branding = context and context.get('inherit_branding')
        if not inherit_branding and context and context.get('inherit_branding_auto'):
            inherit_branding = self.pool['ir.model.access'].check(cr, uid, record._name, 'write', False, context=context)
        translate = context and context.get('edit_translations') and context.get('translatable') and getattr(record._fields[field_name], 'translate', False)
        if inherit_branding or translate: