Headers#get won't return undefined but typed as js.UndefOr

[unarist]

The headers#get method returns a header value or null. So this method won't return undefined. but it's typed with js.UndefOr.
https://developer.mozilla.org/en-US/docs/Web/API/Headers/get

scala-js-dom/src/main/scala/org/scalajs/dom/experimental/Fetch.scala

Line 312 in 5a502ad

def get(name: ByteString): js.UndefOr[ByteString] = js.native