undefined behaviour in json.h using strcat

[Amleto]

http://en.cppreference.com/w/cpp/string/byte/strcat

https://github.com/ipkn/crow/blob/master/include/json.h#L289

throw std::runtime_error(strcat("expected number, got: ", get_type_str(t())));

Passing a literal into strcat destination is undefined behaviour.

[digitalist]

should we patch it already?