fix json loadValue overloading resolution

resolve ambiguity with the following overloads:

template<class T> inline
typename std::enable_if<(std::is_unsigned<T>::value && sizeof(T) == sizeof(uint64_t)), void>::type
loadValue(T & val)

and

template<class T> inline
typename std::enable_if<std::is_signed<T>::value && sizeof(T) == sizeof(int64_t), void>::type
loadValue(T & val)