Support for GCC 9 ?!

Created by: soeth16

Hi at all, In order to compile the framework with GCC 9.2 I made following changes:

src/ArduinoJson/Object/MemberProxy.hpp

...
template <typename TObject, typename TStringRef>
class MemberProxy : public VariantOperators<MemberProxy<TObject, TStringRef> >,
                    public Visitable {
  typedef MemberProxy<TObject, TStringRef> this_type;

 public:
  MemberProxy(const MemberProxy&) = default; // GCC 9
...

src/ArduinoJson/Array/ElementProxy.hpp

...
template <typename TArray>
class ElementProxy : public VariantOperators<ElementProxy<TArray> >,
                     public Visitable {
  typedef ElementProxy<TArray> this_type;

 public:
  ElementProxy(const ElementProxy&) = default; // GCC 9
...

extras/tests/JsonDocument/BasicJsonDocument.cpp:

...
class SpyingAllocator {
 public:
  SpyingAllocator(const SpyingAllocator&) = default; // GCC 9
...