Core API Issue on adding droplet into load balancer

I am reporting issue more on the core API, instead of the ruby interface, as this happens on both ruby and curl.

When I try to add a droplet into a load balancer, using:
```
@client.load_balancers.add_droplets([droplet1_id], id: lb.id)
```

It just returns
```
Error: 422: {"id":"unprocessable_entity","message":"need to provide either a tag or a list of droplets","request_id":"f80013f6-f06b-400f-8591-cb6d5305dbb6"}.
```

Start by reproducing the failure with load_balancers.add_droplets([droplet1_id], id: lb.id), then compare the request produced by the Ruby client with the equivalent curl request. Trace the add_droplets entry point and verify that the API receives a list of droplets; done means adding the droplet succeeds without the 422 response.

Written by the indexing model from the issue text.