type specialize option for an edge case (fix #4930)

[bobzhang]

@obj external f: (~lo: int, ~hi: string=?, unit) => _ = ""

let u = (lo, hi) => f(~lo, ~hi?, ())

Generated code was:

function u(lo, hi) {
  var tmp = {
    lo: lo
  };
  if (hi !== undefined) {
    tmp.hi = Caml_option.valFromOption(hi);
  }
  return tmp;
}

Now:

function u(lo, hi) {
  var tmp = {
    lo: lo
  };
  if (hi !== undefined) {
    tmp.hi = hi;
  }
  return tmp;
}

[bobzhang]

For the record, using spreading operators seem more elegant syntax wise:

return { lo,
              ... (optional1 === undefined ? {}, {optional1 },
              ... (optional2 === undefined ? {}, {optional2},
              ... (optional3 === undefined ? {}, {optional3}
            }

cc @rickyvetter IIRC, the optional parameter is used a lot in react bindings, this could have a large impact on those? https://github.com/rescript-lang/rescript-react/blob/master/src/ReactDOM.res#L57