feat(Dropdown): support MenuItem and Button types

Steps

I'm using a Dropdown as Menu.Item and noticed that the width of the menu item seems off. I checked the Semantic-UI docs (3rd example) and they are using an additional icon class for the menu item if the Dropdown only contains an Icon.

Expected

class="ui dropdown icon item"

Result

class="ui dropdown item"

Workaround

adding className='icon'as additional Dropdownprop

Testcase

http://codepen.io/Chlris-R3/pen/PGQbqZ

• left: without icon class
• right: using className='icon' as additional Dropdownprop

Is this a bug / missing feature / working as intended?