text: Title: CaptureAsImage results a black image for second monitor (maybe re-use pyWin32?)
  Body: In a simple test, using Notepad as an example, CaptureAsImage method results a image file fully black
  as shown below:
  
  ![img](https://user-images.githubusercontent.com/12866789/31677303-16ba6336-b349-11e7-9745-8131a68f5d60.png)
  
  
  The code that I'm using is:
  
  from pywinauto.application import Application
  app = Application().start('notepad.exe')
  app.top_window_().set_focus().CaptureAsImage().save("d:\img.png")
  
  
  Thank you
  
  