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)


return self.copy()
class QWeb(orm.AbstractModel):
    """ Base QWeb rendering engine
    * to customize ``t-field`` rendering, subclass ``ir.qweb.field`` and
      create new models called :samp:`ir.qweb.field.{widget}`
    * alternatively, override :meth:`~.get_converter_for` and return an
      arbitrary model to use as field converter
    Beware that if you need extensions or alterations which could be
    incompatible with other subsystems, you should create a local object
    inheriting from ``ir.qweb`` and customize that.
    """
    _name = 'ir.qweb'
    _void_elements = frozenset([
        u'area', u'base', u'br', u'col', u'embed', u'hr', u'img', u'input',
        u'keygen', u'link', u'menuitem', u'meta', u'param', u'source',
        u'track', u'wbr'])
    _format_regex = re.compile(
        '(?:'
            
            '#\{(.+?)\}'
        ')|(?:'
            
            '\{\{(.+?)\}\}'
        ')')
    def __init__(self, pool, cr):