Valid return values for on_body?

[ronag]

Documented for other methods but not on_body.

https://github.com/nodejs/llhttp/blob/master/src/native/api.h#L40

[indutny]

Would appreciate a PR for that! All llhttp_data_cb should return 0 on success and non-zero value will interrupt the parser and make llparse_execute set error to that value.