When options.fragment is 'body', now will use body.contents from response, rather than the contents of the body's first child.

I couldn't get it pjax working with full html responses.  I just wanted pjax to replace the title and body with the title and the body of the response.  This seems like a simple use case.  

However, in the extractContainer function, if options.fragment === 'body' and the response is a full html response, here's what I was seeing:

$body is set to the inner contents (essentially innerHTML)of the response's body, which is the array of child elements inside body.
$fragment is set to $body, which is misleading because it's actually the inner contents of the body, not the body element itself. 
obj.contents is set to $fragment.contents(), which was, I believe, getting the inner contents of each element in this array.

Instead, in the special case where $fragment is the inner contents of body, you want to just use $fragment itself, not $fragment.contents().