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)


if self._content is None:
            self._content = self.inline or self._fetch_content()
        return self._content
    def _fetch_content(self):
        """ Fetch content from file or database"""
        try:
            self.stat()
            if self._filename:
                with open(self._filename, 'rb') as fp:
                    return fp.read().decode('utf-8')
            else:
                return self._ir_attach['datas'].decode('base64')
        except UnicodeDecodeError:
            raise AssetError('%s is not utf-8 encoded.' % self.name)
        except IOError:
            raise AssetNotFound('File %s does not exist.' % self.name)
        except:
            raise AssetError('Could not get content for %s.' % self.name)
    def minify(self):
        return self.content
    def with_header(self, content=None):
        if content is None:
            content = self.content
        return '\n/* %s */\n%s' % (self.name, content)
class JavascriptAsset(WebAsset):