fix undefined values in objects

[Coobaha]

regression for undefined values in objects. I think it should make Jsonlike key optional, like:

type Obj = {
  a: string | null | undefined
}

type ResultJsonLikeType = {
 a?: string | null
}
declare const obj: Obj;

// works as JSON.strinigfy(obj) will remove undefined values
req.send(obj) // results in a {} response