delete copy constructor of HttpClient

http.h

@@ -22,6 +22,7 @@ namespace http {
   public:
     HttpClient(uv_loop_t *loop);
     HttpClient(HttpClient&&) = delete;
+    HttpClient(HttpClient&) = delete;
     ~HttpClient();
     bool send_request(std::string method, std::string url, HttpOptions opts, ResponseCallback cb);
   private: