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)


html_url_format = '%s'
    def __init__(self, bundle, inline=None, url=None):
        self.id = str(uuid.uuid4())
        self.bundle = bundle
        self.inline = inline
        self.url = url
        self.html_url_args = url
        self.cr = bundle.cr
        self.uid = bundle.uid
        self.registry = bundle.registry
        self.context = bundle.context
        self._content = None
        self._filename = None
        self._ir_attach = None
        name = '<inline asset>' if inline else url
        self.name = "%s defined in bundle '%s'" % (name, bundle.xmlid)
        if not inline and not url:
            raise Exception("An asset should either be inlined or url linked")
    @property
    def html_url(self):