Add default copy constructors/assignment operators to json (de)serializers

Fixes #9002

It worked fine with native gcc 4.7.2. Changes have been tested with gcc version 4.8.1. Seems I should upgrade my compiler suite. I apologize for the inconvenience.

The 'JsonSerializer'/'JsonDeserializer' classes did not have a user proper copy constructor / assignment operator, even through both classes have no members at all. The virtual destructor seems to have prevent the compiler from automatically creating those functions. Strangely the build server does not complain about this, and it uses mingw, doesn't it?