Feature/fix client retries

BaseClient (and all subclasses) had a bug which was overriding a per-instance default value for number of times to retry a failed http connection with a send_request() default value of 0.  Changing the default value of BaseClient.send_request()'s  retries parameter to None in BaseClient lets ProxyClient.send_request()'s logic sort out whether to use the per-instance value or the method parameter.