discount_rate type error in Xeroizer::Record::LineItem#line_amount

[balmoral]

discount_rate if present is a string and needs to be converted to a float when calculating line_amount in a LineItem:

    class LineItem < Base
      def line_amount(summary_only = false)
        return attributes[:line_amount] if summary_only || @line_amount_set

        if quantity && unit_amount
          total = quantity * unit_amount
          if discount_rate
            # CHANGE: discount_rate.to_f
            BigDecimal((total * ((100 - discount_rate.to_f) / 100)).to_s).round(2)
          else
            BigDecimal(total.to_s).round(2)
          end
        end
      end
    end

[CloCkWeRX]

I just merged #397; perhaps you'd like to do a similar PR?