Use Option.apply insted of toOpt in ExtractableJsonAstNode#getAs

`ExtractableJsonAstNode#getAs[A]` has the same problem as https://github.com/json4s/json4s/issues/89
I think `getAs` should behave similar to `extractOpt`.
@dozed `extractOpt` don't return `Some(null)` after #89 was fixed.

```scala
scala> val json = parse(""" {"x": null} """)
json: org.json4s.JValue = JObject(List((x,JNull)))

//the present;
(json \ "x").getAs[String]           //Some(null)
(json \ "x").extractOpt[String]   //None


//this PR
(json \ "x").getAs[String]           //None
```