Return JSON content type in Zulip webhook responses

I suspect that this might have caused the encoding problems.

Author: Kobzol

src/main.rs

@@ -155,6 +155,7 @@ async fn serve_req(
 
         return Ok(Response::builder()
             .status(StatusCode::OK)
+            .header("Content-Type", "application/json")
             .body(Body::from(triagebot::zulip::respond(&ctx, req).await))
             .unwrap());
     }