fix compile error with c++17

CMakeLists.txt

@@ -12,7 +12,7 @@
 
 cmake_minimum_required(VERSION 3.0.0)
 
-project(jsonrpcpp VERSION 1.2.0 LANGUAGES CXX)
+project(jsonrpcpp VERSION 1.2.1 LANGUAGES CXX)
 set(PROJECT_DESCRIPTION "C++ JSON-RPC 2.0 library")
 set(PROJECT_URL "https://github.com/badaix/jsonrpcpp")
 

include/jsonrpcpp.hpp

@@ -3,11 +3,11 @@
      _(  )/ ___) /  \ (  ( \(  _ \(  _ \ / __)( )  ( )  
     / \) \\___ \(  O )/    / )   / ) __/( (__(_ _)(_ _) 
     \____/(____/ \__/ \_)__)(__\_)(__)   \___)(_)  (_)  
-    version 1.2.0
+    version 1.2.1
     https://github.com/badaix/jsonrpcpp
 
     This file is part of jsonrpc++
-    Copyright (C) 2017-2018 Johannes Pohl
+    Copyright (C) 2017-2019 Johannes Pohl
 
     This software may be modified and distributed under the terms
     of the MIT license.  See the LICENSE file for details.
@@ -869,7 +869,7 @@ inline void Error::parse_json(const Json& json)
 		code_ = json["code"];
 		if (json.count("message") == 0)
 			throw RpcException("message is missing");
-		message_ = json["message"];
+		message_ = json["message"].get<std::string>();
 		if (json.count("data") != 0u)
 			data_ = json["data"];
 		else