<span class="Apple-tab-span"> remove

[yiminghe]

Webkit-based browser generates

<span class='Apple-tab-span' style='whitespace:pre'>         </span>

to show tab effect when pasted from somewhere(outlook).

Since ck will collapse extra whitespace except pre, it leaves:

<span class='Apple-tab-span' style='whitespace:pre'> </span>

So there is no reason keeping this span. see attachment patch.

[j.swiderski]

We had similar issue already #4876 but this one treats about removing not needed garbage. I know it won't be an issue in v4 of the editor but IMO we could also do it for v3 with the patch provided by @yiminghe.