Fix MSVC 15.9 (2017) warning C4866

by changing operator[] param type from JSONCPP_STRING to const JSONCPP_STRING& for CharReaderBuilder and StreamWriterBuilder (as it is already in Value).

https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/c4866?view=vs-2017

src/lib_json/json_reader.cpp

@@ -1969,7 +1969,7 @@ bool CharReaderBuilder::validate(Json::Value* invalid) const {
   }
   return 0u == inv.size();
 }
-Value& CharReaderBuilder::operator[](JSONCPP_STRING key) {
+Value& CharReaderBuilder::operator[](const JSONCPP_STRING& key) {
   return settings_[key];
 }
 // static