fix(device): return an error when setting an empty config to BwDevice

Refs: #6

rattan/src/devices/bandwidth.rs

@@ -175,6 +175,12 @@ where
     type Config = BwDeviceConfig<P, Q>;
 
     fn set_config(&self, config: Self::Config) -> Result<(), Error> {
+        if config.bandwidth.is_none() && config.queue_config.is_none() {
+            // This ensures that incorrect HTTP requests will return errors.
+            return Err(Error::ConfigError(
+                "At least one of bandwidth and queue_config should be set".to_string(),
+            ));
+        }
         if let Some(bandwidth) = config.bandwidth {
             if bandwidth > MAX_BANDWIDTH {
                 return Err(Error::ConfigError(
@@ -275,6 +281,7 @@ where
     fn peek(&self) -> Option<&P>;
 }
 
+#[derive(Debug)]
 pub struct InfiniteQueue<P> {
     queue: VecDeque<P>,
 }
@@ -330,6 +337,7 @@ impl FiniteQueueConfig {
     }
 }
 
+#[derive(Debug)]
 pub struct DropTailQueue<P> {
     queue: VecDeque<P>,
     packet_limit: usize,
@@ -390,6 +398,7 @@ where
     }
 }
 
+#[derive(Debug)]
 pub struct DropHeadQueue<P> {
     queue: VecDeque<P>,
     packet_limit: usize,