Fix paper tab disabled

[MarcoUmpierrez]

Hi, I saw that this issue #791 hasn't been fixed yet. Here's a PR to fix it. I've added a test and an example of a disabled tab in the dummy app.

[miguelcobain]

@MarcoUmpierrez could you please change this line to if (!this.get('disabled')) { to conform with the rest of the project's coding style?

[miguelcobain]

@MarcoUmpierrez this looks awesome. Thank you so much.

There's still something that isn't covered that shouldn't be too hard to do.

A tab can be an anchor if an href is passed. If a tab with an href is disabled, we need to disable a link. But how do we do that?
Well, a simple way is to just don't have any href.
So we can have an intermediate value that "maybe" holds the href if not disabled.

attributeBindings: ['maybeHrefhref'],
maybeHref: computed('href', 'disabled', function() {
  if (this.get('href') && !this.get('disabled')) {
    return this.get('href');
  }
}),

Having a test that made sure that no href was rendered for a disabled tab would also be helpful.

[MarcoUmpierrez]

@miguelcobain Added the changes you requested and a test to check if the href is set when the tab is disabled.

[miguelcobain]

Thanks a lot @MarcoUmpierrez !