Saving changed models fails with rails/activeresource master

To reproduce:
```
#Gemfile:
gem 'activeresource', github: 'rails/activeresource', branch: 'master'
gem 'shopify_api', github: 'Shopify/shopify_api', branch: 'master'
```

```
p = ShopifyAPI::Product.find(_existing_product_id_)
p.title = p.title + " changed"
p.save

=>
PUT https://tees-dev.myshopify.com:443/admin/products/288628431.json
INFO -- : --> 400 Bad Request 62 (328.0ms)
INFO -- : Request:
{}
INFO -- : Headers: {"Content-Type"=>"application/json", "X-Shopify-Access-Token"=>"xxxx"}
INFO -- : Response:
{"errors":{"product":"Required parameter missing or invalid"}}
ActiveResource::BadRequest: Failed.  Response code = 400.  Response message = Bad Request.
from bundler/gems/activeresource-23786e2ff935/lib/active_resource/connection.rb:138:in `handle_response'
```

The problem is in the overridden `ActiveResource::Base#encode` method.