screenshots of 500 pages will now show current page instead of previous page

The problem is that in phantomjs only:

```coffeescript
@browser.navigateTo 'http://httpstat.us/200'
@browser.navigateTo 'http://httpstat.us/500'
screenshot = @browser.getScreenshot()
# screenshot is of the /200 page, not /500
```

This PR forces all responses to be 200 (as far as the browser is concerned), but preserves the real http status code in the cookie, where it can be retrieved as normal.

I'm not a fan of this change, but it does fix the problem. I'm open to other ideas, though.