Gremlin.Net: Closed connections should not be re-used

The driver of Gremlin.Net is kept very simle which holds especially true for the ConnectionPool. It simply returns every connection to its pool of usable connections that was disposed by the client. Unfortunately, this also applies in case the submit failed due to an already closed connection which means that the client will get the closed connection back from the ConnectionPool later and continues trying to submit messages over this closed connection.
This can be fixed by checking whether the Connection is still open before adding it back to the ConnectionPool.