Document server_version behavior seen in the wild

I've seen each of these 2 errors from different servers when using the default `server_version` value and the `Viewpoint::EWSClient#folders` method:

```
Viewpoint::EWS::Errors::SoapResponseError: SOAP Error: Message:   Code: 
Viewpoint::EWS::Errors::ResponseError: HTTP Error Code: 400
```

They work with `VERSION_2007` or `VERSION_2007_SP1`, but at least in the case of one server, only `VERSION_2007_SP1` and not `VERSION_2007`. I don't run or know anything about any of the servers I'm targeting, so this PR just documents the behavior I'm seeing.