button: wrong definition of real button

[sipayRT]

Now in templates we check just type modifier:

var modType = ctx.mod('type'),
      isRealButton = !modType || modType === 'submit';

But if we will change in BEMJSON tag, for example, to <div>, variable isRealButton still will be true. It's wrong because we can't add some attributes such as disabled to <div>.

In the attach block it's very important because we use <span> tag instead of <button>. As a result — disabled button in attach block will be disabled only visually for a11y's programs.

[sipayRT]

@veged @dfilatov @narqo @tadatuta @aristov
Are we really need to keep in mind the possibility of changing the tag? Or we shouldn't support it?

[tadatuta]

as we already use different tag in attach I suggest to add the check for tag to isRealButton condition as well