In case anyone washed up this shore looking for how to do this with Poltergeist you just need to pass the full argument:
page.save_screenshot('screen.png', full: true) # If providing a custom file name.
page.save_screenshot(full: true) # Capybara sets a name based on timestamp.
page.save_and_open_screenshot('screen.png', full: true) # Same as save_screenshot.
page.save_and_open_screenshot(full: true) # Same as save_screenshot.
Docs.
Hope it helps!