Issue Description:

The following code handles pipe input.

Based on the documentation, when `availableData` length is 0, it means the end of the pipe has been reached and the read operation is complete. Hover, we seen the handler get called with zero length availableData multiple times, which seems like a bug.

let stdoutPipe = Pipe()
stdoutPipe.fileHandleForReading.readabilityHandler = { (fh : FileHandle) -> Void in
  let data = fh.availableData
  if (data.count == 0) {
    print("finished")
  }
}