submissions-api: hardcode header to application/json

validation/api/src/context.rs

@@ -22,6 +22,7 @@ impl Context{
 	}
 	pub async fn post(&self,url:impl reqwest::IntoUrl,body:impl Into<reqwest::Body>)->Result<reqwest::Response,reqwest::Error>{
 		self.client.post(url)
+    	.header("Content-Type","application/json")
     	.body(body)
 		.send().await
 	}