User can now use a JsonString as a key or a value

src/ArduinoJson/Strings/StringAdapters.hpp

@@ -0,0 +1,36 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2018
+// MIT License
+
+#pragma once
+
+#include "../Memory/MemoryPool.hpp"
+#include "../Polyfills/type_traits.hpp"
+
+namespace ARDUINOJSON_NAMESPACE {
+template <typename>
+struct IsString : false_type {};
+
+template <typename T>
+struct IsString<const T> : IsString<T> {};
+
+template <typename T>
+struct IsString<T&> : IsString<T> {};
+}  // namespace ARDUINOJSON_NAMESPACE
+
+#include "ConstRamStringAdapter.hpp"
+#include "RamStringAdapter.hpp"
+#include "SizedRamStringAdapter.hpp"
+
+#if ARDUINOJSON_ENABLE_STD_STRING
+#include "StlStringAdapter.hpp"
+#endif
+
+#if ARDUINOJSON_ENABLE_ARDUINO_STRING
+#include "ArduinoStringAdapter.hpp"
+#endif
+
+#if ARDUINOJSON_ENABLE_PROGMEM
+#include "FlashStringAdapter.hpp"
+#include "SizedFlashStringAdapter.hpp"
+#endif