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)


self.string = string
    def __html__(self):
        return self.string
    def __str__(self):
        s = self.string
        if isinstance(s, unicode):
            return s.encode('utf-8')
        return s
    def __unicode__(self):
        s = self.string
        if isinstance(s, str):
            return s.decode('utf-8')
        return s
def nl2br(string, options=None):
    """ Converts newlines to HTML linebreaks in ``string``. Automatically
    escapes content unless options['html-escape'] is set to False, and returns
    the result wrapped in an HTMLSafe object.
    :param str string:
    :param dict options:
    :rtype: HTMLSafe
    """
    if options is None: options = {}
    if options.get('html-escape', True):
        string = escape(string)
    return HTMLSafe(string.replace('\n', '<br>\n'))
def get_field_type(field, options):
    """ Gets a t-field's effective type from the field definition and its options """
    return options.get('widget', field.type)
class AssetError(Exception):