Add JsonVariant::link() (resolves #1343)

extras/tests/Cpp11/nullptr.cpp

@@ -43,5 +43,13 @@ TEST_CASE("nullptr") {
 
     variant.clear();
     REQUIRE(variant.is<std::nullptr_t>() == true);
+
+    StaticJsonDocument<128> doc2;
+    doc2["hello"] = "world";
+    variant.link(doc2);
+    REQUIRE(variant.is<std::nullptr_t>() == false);
+
+    doc2.clear();
+    REQUIRE(variant.is<std::nullptr_t>() == true);
   }
 }