Decode escaped Unicode characters like \u00DE (issue #304, PR #791)

test/MixedConfiguration/decode_unicode_0.cpp

@@ -0,0 +1,11 @@
+#define ARDUINOJSON_DECODE_UNICODE 0
+#include <ArduinoJson.h>
+
+#include <catch.hpp>
+
+TEST_CASE("ARDUINOJSON_DECODE_UNICODE == 0") {
+  DynamicJsonDocument doc(2048);
+  DeserializationError err = deserializeJson(doc, "\"\\uD834\\uDD1E\"");
+
+  REQUIRE(err == DeserializationError::NotSupported);
+}