Fix ambiguous function template overload in json.hpp
Fix ambiguous function template overload in json.hpp

There are already the "small signed/unsigned overloads" of loadValue that
are always selected for integers smaller than 64 bits.

The fixed overloads conflicted with the "small signed/unsigned overloads"
if:

- long is smaller than 64 bits
- int32_t is not typedef for long

USCiLab/cereal,USCiLab/cereal,adasworks/cereal,fawkes74/cereal,abutcher-gh/cereal,jkhoogland/cereal,adasworks/cereal,jkhoogland/cereal,fawkes74/cereal,abutcher-gh/cereal