fix(Header): subheader and image shorthand

[levithomason]

The Header icon/image requires the content div. The content div should contain all text.

Image

The image shorthand currently does not wrap the content shorthand in HeaderContent.

Subheader

Header subheaders should be rendered as HeaderContent children. Currently, the subheader shorthand is rendered as a sibling when there is an icon/image. Example here only shows an icon, but the issue exists for subheaders when there is an icon or an image.

<Header icon='user' content='foo' subheader='bar' />

Outputs:

<div class="ui header">
  <i class="user icon"></i>
  <div class="content">foo</div>
  <div class="sub header">bar</div>
</div>

It should output:

<div class="ui header">
  <i class="user icon"></i>
  <div class="content">
    foo
    <div class="sub header">bar</div>
  </div>
</div>

[levithomason]

I've marked this help wanted, but I actually need it now. PR coming.