web-mode-comment-or-uncomment fix

Hi. Lets look at following snippet (underscore is cursor position):
```html
<div>
_  <!-- <p>foo</p> -->
<div>
```
Calling web-mode-comment-or-uncomment function in such case resulted in commenting out your own comment twice (and not uncommenting existing comment):
```html
<div>
_  <!-- <!-- <p>foo</p> --> -->
<div>
```
I don't think it's the most desirable behaviour.
Calling web-mode-comment-or-uncomment function in front of a comment - uncomments existing comment after applying the this patch.
Thanks.