Fix no instance of overloaded function... on IAR (fixes #2001)

Ported from 6ed87029e2

src/ArduinoJson/Variant/JsonVariant.hpp

@@ -45,7 +45,7 @@ bool copyVariant(JsonVariant dst, JsonVariantConst src);
 
 template <>
 struct Converter<JsonVariant> : private detail::VariantAttorney {
-  static void toJson(JsonVariant src, JsonVariant dst) {
+  static void toJson(JsonVariantConst src, JsonVariant dst) {
     copyVariant(dst, src);
   }
 

src/ArduinoJson/Variant/VariantRefBase.hpp

@@ -60,7 +60,8 @@ class VariantRefBase : public VariantTag {
     return Converter<T>::fromJson(getVariant());
   }
 
-  template <typename T>
+  template <typename T,
+            typename = typename enable_if<!is_same<T, TDerived>::value>::type>
   FORCE_INLINE operator T() const {
     return as<T>();
   }