Found issue when using Credentials

[KevinNisbet]

Hey,

I was doing a little playing around with you're code, and when using the Network Credentials, it actually looses the credentials when it remakes the RestClient in the TryGetMember Function in RestClient.cs.

To fix this, I added the following:
if (_credentials != null)
operationGroupClient._credentials = _credentials;

The function as a whole:

      public override bool TryGetMember(GetMemberBinder binder, out object result) {
        if (_parameters == null) {
            _parameters = new Dictionary<string, object>();
        }

        object value;
        if (_parameters.TryGetValue(binder.Name, out value)) {
            result = value;
            return true;
        }

        string operationGroup = binder.Name;
        if (_operationGroup != null) {
            operationGroup = _operationGroup + "." + operationGroup;
        }

        RestClient operationGroupClient =
            new RestClient(_uriFormat, _service, operationGroup, _parameters);

        if (_credentials != null)
            operationGroupClient._credentials = _credentials;

        result = operationGroupClient;
        return true;
    }

Just thought I'd contribute back.

Thank You,