Fixed "deprecated-copy" warning on GCC 9 (fixes #1184)

extras/tests/JsonDocument/BasicJsonDocument.cpp

@@ -11,6 +11,7 @@ using ARDUINOJSON_NAMESPACE::addPadding;
 
 class SpyingAllocator {
  public:
+  SpyingAllocator(const SpyingAllocator& src) : _log(src._log) {}
   SpyingAllocator(std::ostream& log) : _log(log) {}
 
   void* allocate(size_t n) {