Fixes creation of TaxRates

Before, there was no primary key defined for TaxRates, so attempting to
create one resulted in the following exception:

```
TypeError: nil is not a symbol
xeroizer/lib/xeroizer/record/base.rb:52:in `[]'
xeroizer/lib/xeroizer/record/model_definition_helper.rb:88:in `id'
xeroizer/lib/xeroizer/record/base.rb:78:in `new_record?'
xeroizer/lib/xeroizer/record/base.rb:101:in `save'
```

This stemmed from being unable to query whether the TaxRate being
created was truly a `new_record?` or not.
Merge pull request #163 from chargify/tax_rates

Fixes Creation of TaxRates