paginated_resource raises error with no api key set

```
client = DropletKit::Client.new(access_token: nil)
client.droplets.all.each do |droplet|
puts droplet.name
end
=> TypeError: no implicit conversion of String into Array
from /Users/jadems/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/droplet_kit-1.2.3/lib/droplet_kit/paginated_resource.rb:68:in `retrieve'
from /Users/jadems/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/droplet_kit-1.2.3/lib/droplet_kit/paginated_resource.rb:60:in `fetch_next_page'
from /Users/jadems/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/droplet_kit-1.2.3/lib/droplet_kit/paginated_resource.rb:26:in `each'
```

I had a misconfigured environment and spent a few minutes trying to figure out why I was getting this. It might be helpful to handle authentication errors more gracefully.

Start with lib/droplet_kit/paginated_resource.rb, especially retrieve around line 68 and fetch_next_page around line 60, then reproduce the issue using a client with a nil access token and droplets.all. Trace how the authentication failure becomes a TypeError; done means the misconfigured client reports an understandable authentication error instead.

Written by the indexing model from the issue text.