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
return self.checksum[0:7]
    @lazy_property
    def checksum(self):
        """
        Not really a full checksum.
        We compute a SHA1 on the rendered bundle + max linked files last_modified date
        """
        check = self.html + str(self.last_modified)
        return hashlib.sha1(check).hexdigest()
    def clean_attachments(self, type):
        """ Takes care of deleting any outdated ir.attachment records associated to a bundle before
        saving a fresh one.
        When `type` is css we need to check that we are deleting a different version (and not *any*
        version) because css may be paginated and, therefore, may produce multiple attachments for
        the same bundle's version.
        When `type` is js we need to check that we are deleting a different version (and not *any*
        version) because, as one of the creates in `save_attachment` can trigger a rollback, the
        call to `clean_attachments ` is made at the end of the method in order to avoid the rollback
        of an ir.attachment unlink (because we cannot rollback a removal on the filestore), thus we
        must exclude the current bundle.
        """
        ira = self.registry['ir.attachment']
        domain = [
            ('url', '=like', '/web/content/%-%/{0}%.{1}'.format(self.xmlid, type)),  
            '!', ('url', '=like', '/web/content/%-{}/%'.format(self.version))
        ]
        attachment_ids = ira.search(self.cr, openerp.SUPERUSER_ID, domain, context=self.context)
        return ira.unlink(self.cr, openerp.SUPERUSER_ID, attachment_ids, context=self.context)
    def get_attachments(self, type, inc=None):