Fix #827: Implement java.net.URI

Seriously, a `def` with 2 parameters with the same name as a `var` which has an `apply` method with 1 parameter?

Make the field `fld` hidden as `_fld` and defined another method
```scala
@inline private def fld(idx: Int): js.UndefOr[String] =
  _fld(idx)
```