subset() Error message wrong when duplicate parameters

If `info` response has duplicate parameter names, the error [`Order of requested parameters does not match order of parameters in server info metadata.`](https://github.com/hapi-server/client-python/blob/master/hapiclient/hapi.py#L58) is triggered.

The clients typically do minimal error checking because the server is assumed to emit valid metadata. This is an error we may want to catch.