Fix subscription updates when subcription has add_ons

[bhelx]

Addresses #240
Reported By: @ScottSpittle

We are seeing an issue with updating subscriptions containing add ons. Using the updateImmediately method was generating some xml which was trying to update immutable fields for the subscription add ons on the backend. The update payload was looking something like this:

<?xml version="1.0" encoding="UTF-8"?>
<subscription>
   <quantity>2</quantity>
   <timeframe>now</timeframe>
   <subscription_add_ons>
      <subscription_add_on>
         <add_on_code>marketing_emails</add_on_code>
         <add_on_type>fixed</add_on_type>
         <quantity>1</quantity>
         <unit_amount_in_cents>5</unit_amount_in_cents>
      </subscription_add_on>
   </subscription_add_ons>
</subscription>

And the resulting error:

<?xml version="1.0" encoding="UTF-8"?>
<error>
  <symbol>invalid_xml</symbol>
  <description>The provided XML was invalid.</description>
  <details>Unacceptable tag &lt;add_on_type&gt;</details>
</error>

I added some code to ignore the immutable fields on update but I think this is a larger problem around changes to arrays not being tracked. We shouldn't be rendering the subscription add ons unless they have changed.

So while this should be a fine solution to this problem, I think a follow up should be made to investigate how we handle tracking of changes on arrays of objects.

[drewish]

+1