Failing test: trigger updates correctly when subprop is initialized with null

[ddworak]

https://github.com/UdashFramework/udash-core/commit/2f671cfa7bcad8150ee9f7517e3027d51fb3ea74

[ddworak]

Related #664, verify FileInput.single usage

[catap]

@ddworak from my point of view FileInput.single should be implemented as

  def single(selectedFile: Property[Option[File]])(inputName: String, inputModifiers: Modifier*): InputBinding[JSInput] = {
    apply(selectedFile.bitransformToSeq(_.toSeq)(_.headOption), false.toProperty)(inputName, inputModifiers)
  }

anyway, I can miss some way to use it

[ddworak]

Sure, single should be fixed in the long run - I was just suggesting a workaround.