GenericValue: avoid memset/memcpy(this,...)

To avoid writing outside of the current GenericValue object and to
follow the C++ standard more closely, this patch drops the
memset/memcpy(this,...) occurences in GenericValue in favour of explicit
initialisations/assignments of the data_ and flag_ members.

https://code.google.com/p/rapidjson/issues/detail?id=11
https://github.com/pah/rapidjson/commit/7475a969