Fixed exclude getBoundingClientRect on IE8

[adrianolek]

Webdrivercss crashes when using excludes on IE8.
On IE8 the object returned by getBoundingClientRect lacks width and height properties.
The PR fixes this issue.

[christian-bromann]

hey @adrianolek,
thanks for the PR. Can we be 100% sure about elemRect.left + elemRect.width === elemRect.right? This was the reason why I did that in the first place. I mean it should be the same, right? I will check if I can find something about that and if there are no problems with that I will merge it.
Thanks!

[adrianolek]

Seems like they are. I've checked it this way:
https://gist.github.com/adrianolek/35cf7db4b7f92c20a7d6

Besides this IE8 issue is already handled similarily here.

Also there's a test for excluding elements, which still passes.

And from https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIDOMClientRect

height | Height of the rectangle box (This is identical to bottom minus top).
width | Width of the rectangle box (This is identical to right minus left).

[christian-bromann]

Oh nice, thanks for checking! Merging