Issue with text decoration

[PersianBrat]

Taking <span style="text-decoration: line-through;">text</span> as the old string and text as the new, I get the following as the result:

<span style="text-decoration: line-through;">text</span>

Which is the same as the old text and doesn't show the difference. So, I think it should actually be like the following:

<span style="text-decoration: line-through;"><del class='diffmod'>text</del></span><ins class='diffmod'>text</ins>

[PersianBrat]

I've noticed adding/removing tags like <b> also don't add <del> to the diff result. But it adds <ins> tag. So, the same thing can happen for <span> tag I suppose.

[PersianBrat]

I did some changes to make the behaviour the same for <span> tag: #59