add non-const versions of get(size_t) / get(const std::string&) (cherry-picked from 002a2dd by ptarabbia)
add non-const versions of get(size_t) / get(const std::string&) (cherry-picked from 002a2dd by ptarabbia)

turenar/picojson,kazuho/picojson,Banchi0123/picojson,turenar/picojson,pfultz2/picojson,kazuho/picojson,Banchi0123/picojson,karya0/picojson,pfultz2/picojson,karya0/picojson