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)


import math
import os
import re
import sys
import textwrap
import uuid
from subprocess import Popen, PIPE
from urlparse import urlparse
import babel
import babel.dates
import werkzeug
from lxml import etree, html
from PIL import Image
import psycopg2
import openerp.http
import openerp.tools
from openerp.tools.func import lazy_property
import openerp.tools.lru
from openerp.exceptions import QWebException
from openerp.fields import Datetime
from openerp.http import request
from openerp.tools.safe_eval import safe_eval as eval
from openerp.osv import osv, orm, fields
from openerp.tools import html_escape as escape
from openerp.tools.misc import find_in_path
from openerp.tools.translate import _
from openerp.modules.module import get_resource_path
_logger = logging.getLogger(__name__)
MAX_CSS_RULES = 4095
class QWebTemplateNotFound(QWebException):