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)


""" Used to convert a t-field specification into an output HTML field.
    :meth:`~.to_html` is the entry point of this conversion from QWeb, it:
    * converts the record value to html using :meth:`~.record_to_html`
    * generates the metadata attributes (``data-oe-``) to set on the root
      result node
    * generates the root result node itself through :meth:`~.render_element`
    """
    _name = 'ir.qweb.field'
    def attributes(self, cr, uid, field_name, record, options,
                   source_element, g_att, t_att, qweb_context,
                   context=None):
        """ attributes(cr, uid, field_name, record, options, source_element, g_att, t_att, qweb_context, context=None)
        Generates the metadata attributes (prefixed by ``data-oe-`` for the
        root node of the field conversion. Attribute values are escaped by the
        parent.
        The default attributes are:
        * ``model``, the name of the record's model
        * ``id`` the id of the record to which the field belongs
        * ``field`` the name of the converted field
        * ``type`` the logical field type (widget, may not match the field's
          ``type``, may not be any Field subclass name)
        * ``translate``, a boolean flag (``0`` or ``1``) denoting whether the
          field is translatable