I think these two lines may become common source of errors if white space included in beginning or end of the JSON string:
eosio_assert(json[0] == '{', "payload must be json");
eosio_assert(json[json.size()-1] == '}', "payload must be json");

Since you are not doing true JSON validation - you may as well remove those lines.
It would be good to include full JSON validation - however that will require adding a new dependency to your contract file:
There are many JSON validation libraries available. We need to check if EOSIO code is already using one and reuse the same one.