fix: default content type

apps/monitor/src/runner.rs

@@ -300,14 +300,19 @@ fn run_check_blocking(
         .method(method)
         .uri(url);
 
+    let mut has_content_type = false;
     if let Some(hdrs) = headers {
         for (k, v) in hdrs {
+            if k.eq_ignore_ascii_case("content-type") { has_content_type = true; }
             builder = builder.header(k.as_str(), v.as_str());
         }
     }
 
     let result = match body {
         Some(b) => {
+            if !has_content_type {
+                builder = builder.header("Content-Type", "application/json");
+            }
             let req = builder.body(b.as_bytes()).map_err(|e| e.to_string())?;
             agent.run(req)
         }