Added Default Generic Param to HttpClient

[tadeohepperle]

fixes #1096

Added Default Generic Param like so:

/// JSON-RPC HTTP Client that provides functionality to perform method calls and notifications.
#[derive(Debug, Clone)]
pub struct HttpClient<S = HttpBackend> {
	/// HTTP transport client.
	transport: HttpTransportClient<S>,
	/// Request timeout. Defaults to 60sec.
	request_timeout: Duration,
	/// Request ID manager.
	id_manager: Arc<RequestIdManager>,
}

[niklasad1]

👍