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
rx_indent = re.compile(r'^( +|\t+)', re.M)
    indent = None
    reindent = '    '
    def get_source(self):
        content = textwrap.dedent(self.inline or self._fetch_content())
        def fix_indent(m):
            
            ind = m.group()
            if self.indent is None:
                self.indent = ind
                if self.indent == self.reindent:
                    
                    raise StopIteration()
            return ind.replace(self.indent, self.reindent)
        try:
            content = self.rx_indent.sub(fix_indent, content)
        except StopIteration:
            pass
        return "/*! %s */\n%s" % (self.id, content)
    def get_command(self):
        try:
            sass = find_in_path('sass')
        except IOError:
            sass = 'sass'
        return [sass, '--stdin', '-t', 'compressed', '--unix-newlines', '--compass',
                '-r', 'bootstrap-sass']
class LessStylesheetAsset(PreprocessedCSS):