adapts to how vat_rates should be sent

[simonneutert]

vat_rate (for a line_item) should be sent as decimal.

Though it will work with a proper formatted String, too 🤷‍♂️

Please, see changes in this PR for implementation details.

Test

I should rerecord the ver cassettes, but decided to do it in a future release, as the implementation is still working as is.

Before

{
  vat_rate: "19%"
}

After

{
  vat_rate: 0.19
}