Serialize() doesn't fully preserve native types

[adityathebe]

We use jp.Walk to preserve the types for all time.Duration & time.Time. However, jp.Walk doesn't walk through an array.

Example:

{
  "a": "b",
  "c": [
    {
      "d": time.Time{},
      "e": time.Time{}
    }
  ]
}

even though d and e are of time.Time, the callback func in jp.Walk never sees them.