Update POS resource. Returns the raw response object.

        Arguments:
            pos_id:
                POS id as chosen on registration
            name:
                Human-readable name of the POS, used for displaying payment
                request origin to end user
            pos_type:
                POS type
            location:
                Merchant location

def update_pos(self, pos_id, name, pos_type, location=None):
        """Update POS resource. Returns the raw response object.

        Arguments:
            pos_id:
                POS id as chosen on registration
            name:
                Human-readable name of the POS, used for displaying payment
                request origin to end user
            pos_type:
                POS type
            location:
                Merchant location
        """
        arguments = {'name': name,
                     'type': pos_type,
                     'location': location}
        return self.do_req('PUT',
                           self.merchant_api_base_url + '/pos/' +
                           pos_id + '/', arguments)