Impacted versions: ODOO 10

Problem.
If i print a invoice with a negative amount, the pdf shows a vertical bar instead of a minus symbol.
![image](https://cloud.githubusercontent.com/assets/28800797/26237253/b07fadb0-3c74-11e7-86c2-c8688d82b5a5.png)

I have tried to install a new pdf reader, but with no result.
openerp/addons/base/ir/ir_qweb.py
def get_command(self):
        try:
            if os.name == 'nt':
                lessc = find_in_path('lessc.cmd')
            else:
                lessc = find_in_path('lessc')
        except IOError:
            lessc = 'lessc'
        lesspath = get_resource_path('web', 'static', 'lib', 'bootstrap', 'less')
        return [lessc, '-', '--no-js', '--no-color', '--include-path=%s' % lesspath]
def rjsmin(script):
    """ Minify js with a clever regex.
    Taken from http://opensource.perlig.de/rjsmin
    Apache License, Version 2.0 """
    def subber(match):
        """ Substitution callback """
        groups = match.groups()
        return (
            groups[0] or
            groups[1] or
            groups[2] or
            groups[3] or
            (groups[4] and '\n') or