scrollToBottom clipRect error

If i scrollToBottom with casper API in onReady.js when backstopJS capturing page the clipRect height is wrong.

`casper.scrollToBottom();`

`clipRect {"height":427,"left":0,"top":473,"width":1440}`

If after scroll i take a custom capture in the scripts:
`casper.scrollToBottom();
casper.capture("#siteFooter");`

`clipRect {"height":427,"left":0,"top":2758,"width":1440}`

the first capture is wrong but the second is correct.

Thanks!