Dont set Item#rate_percent under the hood

This can cause unexpected results. For example: the Item#unit_price
would not persist in quickbooks as expected because the gem would send a
request with RatePercent: 0.0. That would affect the unit_price setting
it to nothing (0)

Not 100% sure on this but I believe all BigDecimal attributes could take
advantage of this to_xml_big_decimal method introduced here.

Hope it makes sense. Only question pops up here is what if the user wants to set / erase those values explicitly like `item.rate_percent = nil`. Either way I think currently the gem also doesn't support that by calling `to_f`. Let me know if you'd like the other Item decimal attributes to use that method. QtyOnHand is also set to 0.0 for example I just didn't change it yet because I didn't have any trouble with it so far.