Fixed assignment of JsonDocument to JsonVariant (issue #1023)

src/ArduinoJson/Document/JsonDocument.hpp

@@ -278,6 +278,10 @@ class JsonDocument : public Visitable {
     _data.remove(adaptString(key));
   }
 
+  FORCE_INLINE operator VariantConstRef() const {
+    return VariantConstRef(&_data);
+  }
+
  protected:
   JsonDocument(MemoryPool pool) : _pool(pool) {
     _data.setNull();

src/ArduinoJson/Variant/VariantRef.hpp

@@ -222,6 +222,7 @@ class VariantRef : public VariantRefBase<VariantData>,
   // set(ArrayConstRef)
   // set(ObjectRef)
   // set(ObjecConstRef)
+  // set(const JsonDocument&)
   template <typename TVariant>
   typename enable_if<IsVisitable<TVariant>::value, bool>::type set(
       const TVariant &value) const;