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)


pass
def raise_qweb_exception(etype=None, **kw):
    if etype is None:
        etype = QWebException
    orig_type, original, tb = sys.exc_info()
    try:
        raise etype, original, tb
    except etype, e:
        for k, v in kw.items():
            e.qweb[k] = v
        
        e.qweb['cause'] = original
        raise
def _build_attribute(name, value):
    value = escape(value)
    if isinstance(name, unicode): name = name.encode('utf-8')
    if isinstance(value, unicode): value = value.encode('utf-8')
    return ' %s="%s"' % (name, value)
class FileSystemLoader(object):
    def __init__(self, path):
        
        self.path = path
        self.doc = etree.parse(path).getroot()
    def __iter__(self):
        for node in self.doc:
            name = node.get('t-name')
            if name:
                yield name
    def __call__(self, name):