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
size = len(enum)
            copy_qwebcontext["%s_size" % varname] = size
        copy_qwebcontext["%s_all" % varname] = enum
        ru = []
        for index, (item, value) in enumerate(self._iterate(enum)):
            copy_qwebcontext.update({
                varname: item,
                '%s_value' % varname: value,
                '%s_index' % varname: index,
                '%s_first' % varname: index == 0,
            })
            if size is not None:
                copy_qwebcontext['%s_last' % varname] = index + 1 == size
            if index % 2:
                copy_qwebcontext.update({
                    '%s_parity' % varname: 'odd',
                    '%s_even' % varname: False,
                    '%s_odd' % varname: True,
                })
            else:
                copy_qwebcontext.update({
                    '%s_parity' % varname: 'even',
                    '%s_even' % varname: True,
                    '%s_odd' % varname: False,
                })
            ru.append(self.render_element(element, template_attributes, generated_attributes, copy_qwebcontext))
        for k in qwebcontext.keys():
            qwebcontext[k] = copy_qwebcontext[k]
        return "".join(ru)
    def render_tag_if(self, element, template_attributes, generated_attributes, qwebcontext):