Instead of always just shallowly referencing the potentially allocated
strings when calling the Handler::String function, request a copy in
case the string has been allocated from an Allocator before.

This is necessary to avoid double free()s of the string memory,
especially when using the Handler to create a deep copy of a Value.

The explicit comparison against '0' is done to suppress the warning
C4800 on MSVC, see pah/rapidjson#5.