OCurl or ocaml-curl 0.5.3 has a bug around set_postfields
A personal memo. Probably useful for the others met the same problem.
Symptom: POST data set by set_postfields are often sent with garbages at the end.
Cause: a typical bug of forgetting copying \0 in C code
Workaround: In 0.5.3, set_postfieldsize explicitly, with String.length