Repairs InvalidCastException on method RestClientExtensions.ExecuteDynamic

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);
``` 

this PR closes #536