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
diff --git a/include/json/reader.h b/include/json/reader.h
index b700d95..9a7fba2 100644
--- a/include/json/reader.h
+++ b/include/json/reader.h
@@ -358,7 +358,7 @@
/** A simple way to update a specific setting.
*/
- Value& operator[](JSONCPP_STRING key);
+ Value& operator[](const JSONCPP_STRING& key);
/** Called by ctor, but you can use this to reset settings_.
* \pre 'settings' != NULL (but Json::null is fine)