Add unit tests to raise coverage score

Signed-off-by: YUAN LYU <lyuyuan92@gmail.com>

src/common/mod.rs

@@ -522,6 +522,14 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_display_route_error() {
+        assert_eq!(
+            format!("{}", RouteError::HandlerExist("test".to_string())),
+            "handler for test already exists"
+        );
+    }
+
     #[test]
     fn test_method_to_str() {
         let val = Method::Get;