server: use default API version if not specified

src/server.rs

@@ -75,8 +75,13 @@ impl IntoResponse for ApiError {
 
 type ApiResult<T> = Result<T, ApiError>;
 
+fn default_api_version() -> String {
+    "4".to_owned()
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Deserialize)]
 struct ApiVersion {
+    #[serde(default = "default_api_version")]
     api_version: String,
 }