avhttp fix
add #include <boost/json_create_escapes_utf8.hpp>

if some how <boost/json_create_escapes_utf8.hpp> is not included
everywhere that use write_json, the weak template symble that priveded
by server.cpp will just be ignored by linker, because other people is
privideing another symble, but that version does not use the one
overloaded by <boost/json_create_escapes_utf8.hpp>