Fixed error "No such file or directory WString.h" (fixes #1381)

extras/tests/Helpers/api/Stream.h

@@ -0,0 +1,14 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2020
+// MIT License
+
+#pragma once
+
+// Reproduces Arduino's Stream class
+class Stream  // : public Print
+{
+ public:
+  virtual ~Stream() {}
+  virtual int read() = 0;
+  virtual size_t readBytes(char *buffer, size_t length) = 0;
+};