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
if "value" in template_attributes:
            qwebcontext[template_attributes["set"]] = self.eval_object(template_attributes["value"], qwebcontext)
        elif "valuef" in template_attributes:
            qwebcontext[template_attributes["set"]] = self.eval_format(template_attributes["valuef"], qwebcontext)
        else:
            qwebcontext[template_attributes["set"]] = self.render_element(element, template_attributes, generated_attributes, qwebcontext)
        return ""
    def render_tag_field(self, element, template_attributes, generated_attributes, qwebcontext):
        """ eg: <span t-record="browse_record(res.partner, 1)" t-field="phone">+1 555 555 8069</span>"""
        node_name = element.tag
        assert node_name not in ("table", "tbody", "thead", "tfoot", "tr", "td",
                                 "li", "ul", "ol", "dl", "dt", "dd"),\
            "RTE widgets do not work correctly on %r elements" % node_name
        assert node_name != 't',\
            "t-field can not be used on a t element, provide an actual HTML node"
        record, field_name = template_attributes["field"].rsplit('.', 1)
        record = self.eval_object(record, qwebcontext)
        field = record._fields[field_name]
        foptions = self.eval_format(template_attributes.get('field-options') or '{}', qwebcontext)
        options = json.loads(foptions)
        field_type = get_field_type(field, options)
        converter = self.get_converter_for(field_type)
        return converter.to_html(qwebcontext.cr, qwebcontext.uid, field_name, record, options,
                                 element, template_attributes, generated_attributes, qwebcontext, context=qwebcontext.context)
    def get_converter_for(self, field_type):