Refactor send to fallback to default sending params

Refactored the send method in SpontaneousPayment to combine
user-set SendingParameters with default values from Config.
If the user sets any parameter to None, the related default
value is used.

src/payment/spontaneous.rs

@@ -43,7 +43,6 @@ impl SpontaneousPayment {
 	}
 
 	/// Send a spontaneous, or "keysend", payment given an amount in millisatoshi, node ID, and optional [`SendingParameters`].
-
 	pub fn send(
 		&self, amount_msat: u64, node_id: PublicKey, sending_parameters: Option<SendingParameters>,
 	) -> Result<PaymentId, Error> {
@@ -70,13 +69,38 @@ impl SpontaneousPayment {
 			amount_msat,
 		);
 
-		if let Some(payment_params) = sending_parameters {
-			payment_params
-				.max_total_routing_fee_msat
-				.map(|fee| route_params.max_total_routing_fee_msat = Some(fee));
-			payment_params
-				.max_total_cltv_expiry_delta
-				.map(|delta| route_params.payment_params.max_total_cltv_expiry_delta = delta);
+		if let Some(user_set_params) = sending_parameters {
+			if let Some(mut default_params) =
+				self.config.sending_parameters_config.as_ref().cloned()
+			{
+				default_params.max_total_routing_fee_msat = user_set_params
+					.max_total_routing_fee_msat
+					.or(default_params.max_total_routing_fee_msat);
+				default_params.max_total_cltv_expiry_delta = user_set_params
+					.max_total_cltv_expiry_delta
+					.or(default_params.max_total_cltv_expiry_delta);
+				default_params.max_path_count =
+					user_set_params.max_path_count.or(default_params.max_path_count);
+				default_params.max_channel_saturation_power_of_half = user_set_params
+					.max_channel_saturation_power_of_half
+					.or(default_params.max_channel_saturation_power_of_half);
+
+				route_params.max_total_routing_fee_msat = default_params.max_total_routing_fee_msat;
+				route_params.payment_params.max_total_cltv_expiry_delta =
+					default_params.max_total_cltv_expiry_delta.unwrap_or_default();
+				route_params.payment_params.max_path_count =
+					default_params.max_path_count.unwrap_or_default();
+				route_params.payment_params.max_channel_saturation_power_of_half =
+					default_params.max_channel_saturation_power_of_half.unwrap_or_default();
+			}
+		} else if let Some(default_params) = &self.config.sending_parameters_config {
+			route_params.max_total_routing_fee_msat = default_params.max_total_routing_fee_msat;
+			route_params.payment_params.max_total_cltv_expiry_delta =
+				default_params.max_total_cltv_expiry_delta.unwrap_or_default();
+			route_params.payment_params.max_path_count =
+				default_params.max_path_count.unwrap_or_default();
+			route_params.payment_params.max_channel_saturation_power_of_half =
+				default_params.max_channel_saturation_power_of_half.unwrap_or_default();
 		}
 
 		let recipient_fields = RecipientOnionFields::spontaneous_empty();