Fix call of overloaded 'swap(...)' is ambiguous (fixes #1678)

extras/tests/JsonDocument/swap.cpp

@@ -0,0 +1,13 @@
+#include <ArduinoJson.h>
+
+#include <catch.hpp>
+#include <utility>
+
+using namespace std;
+
+TEST_CASE("std::swap") {
+  SECTION("DynamicJsonDocument*") {
+    DynamicJsonDocument *p1, *p2;
+    swap(p1, p2);  // issue #1678
+  }
+}