Use Header.Set() to override the existing header value (#1032)

Header.Add() appends string if there are same header key exists. Use Header.Set() to prevent this.

environments/fetcher/fetcher.go

@@ -482,7 +482,7 @@ func (fetcher *Fetcher) UploadHandler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	log.Println("Completed upload request")
-	w.Header().Add("Content-Type", "application/json")
+	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusOK)
 	w.Write(rBody)
 }