Add tax_code to plan, add on and adjustment

[judith]

Used in tax calculations for VAT 2015 and Avalara integration taxes only.

Valid tax_code values for VAT 2015 taxes are unknown, digital and physical. Valid tax_code values for Avalara integration taxes can be found in the Avalara documentation.

Approvers: @drewish

Tests:
On a site with VAT 2015 and Avalara integration taxes enabled:

• Create/Update a plan, add_on and adjustment with a valid tax_code and verify there isn't an error
• Get the created/updated plan, add_on and adjustment and verify that the tax_code is returned in the results
• Attempt to create/update a plan, add_on and adjustment with an invalid tax_code and verify that an error is returned in the results
  On a site without VAT 2015 and Avalara integration taxes enabled:
• Get a plan, add_on and adjustment and verify that the tax_code is not returned in the results
• Attempt to create/update a plan, add_on and adjustment with a tax_code and verify that an error is returned in the results

[drewish]

Could we get that property added to the specs to demo how the serialization should work?

[judith]

I am hesitant to add it as it is not applicable to the vast majority of users and do not want to confuse anyone by drawing attention to it...especially since sending it when those taxes are not enabled will result in an error. If those taxes are not enabled, the tax_code attribute will not appear in the data returned from the API call.

[drewish]

+1