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
uid = openerp.SUPERUSER_ID
        preprocessed = True
        for atype in (SassStylesheetAsset, LessStylesheetAsset):
            outdated = False
            assets = dict((asset.html_url, asset) for asset in self.stylesheets if isinstance(asset, atype))
            if assets:
                assets_domain = [('url', 'in', assets.keys())]
                ira_ids = self.registry['ir.attachment'].search(self.cr, uid, assets_domain)
                ira_records = self.registry['ir.attachment'].browse(self.cr, uid, ira_ids)
                for ira_record in ira_records:
                    asset = assets[ira_record.url]
                    if asset.last_modified > Datetime.from_string(ira_record['__last_update']):
                        outdated = True
                        break
                    if asset._content is None:
                        asset._content = ira_record.datas and ira_record.datas.decode('base64').decode('utf8') or ''
                        if not asset._content and ira_record.file_size > 0:
                            asset._content = None 
                if any(asset._content is None for asset in assets.itervalues()):
                    outdated = True
                if outdated:
                    if ira_ids:
                        self.registry['ir.attachment'].unlink(self.cr, uid, ira_ids)
                    preprocessed = False
        return preprocessed
    def preprocess_css(self, debug=False):