fix: incorrect json response content type

tests/utils.rs

@@ -0,0 +1,9 @@
+/// Check if the response is a json response
+#[cfg(feature = "into_json")]
+pub async fn check_json(response: reqwest::Response) {
+    assert_eq!(
+        response.headers()[axum::http::header::CONTENT_TYPE],
+        axum::http::HeaderValue::from_static(mime::APPLICATION_JSON.as_ref())
+    );
+    assert!(response.json::<serde_json::Value>().await.is_ok());
+}