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

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?

Edit: I fail to reproduce the original error, so this might not be it. Native gcc 4.7.2 and mingw gcc 4.8.1 compile fine with *and* without this change.