Extension method RestClientExtensions.ExecuteDynamic throws InvalidCastException 

Hi, method RestClientExtensions.ExecuteDynamic throws InvalidCastException : Unable to cast object of type 'RestSharp.RestResponse' to type 'RestSharp.RestResponse`1[System.Object]'. 
on line 

```
var generic = (RestResponse<dynamic>)response;
```

Using generic Execute method repairs problem

```
var response = client.Execute<dynamic>(request);
```
