Don't cache Struct classes 

I have a script for switching failover ripe ips. It tries to select the active and inactive frontend. 

I'm using this method to find out which is active and inactive:

```
dedicatedFailoverRipeList(frontend_hostname)
```

The problem arises when I execute this for inactive frontend, as OVH api will return no attributes and when executing this method again for active frontend will return attributes but it will raise the following error:

```
ovh_savoni-1.54.0/lib/ovh_savoni/response_builder.rb:30:in `initialize': struct size differs (ArgumentError)
```