Add tests for commonly deleted objects, and metafield tests, fix existing test

### Problems 

1. Currently we don't have many tests for the deletion of some frequently deleted objects (so partners don't have examples for the deletion syntax -- as seen in issue https://github.com/Shopify/shopify_api/issues/138)
2. We don't have tests for Metafields (there are dependent tests in `product_test.rb`)
3. In [`order_risk_test`](https://github.com/Shopify/shopify_api/blob/master/test/order_risk_test.rb#L31-L39), two tests mistakenly have the same name. 

### Solutions
1. Look in the EU forum for common requests about deletion --> Add deletion tests in `image_test.rb`, `order_test.rb` and `varient_test.rb`. 
2. Add `metafield_test.rb`.
3. Change name to update. 

@csaunders @jamesmacaulay Please review. 
cc: @pickle27