Always fall back to $body if no fragment specified

Since we're now testing whether `<html>` exists [here](https://github.com/defunkt/jquery-pjax/blob/master/jquery.pjax.js#L573-L579):

```javascript
  // Attempt to parse response html into elements
  if (/<html/i.test(data)) {
    var $head = $($.parseHTML(data.match(/<head[^>]*>([\s\S.]*)<\/head>/i)[0]))
    var $body = $($.parseHTML(data.match(/<body[^>]*>([\s\S.]*)<\/body>/i)[0]))
  } else {
    var $head = $body = $($.parseHTML(data))
  }
```

There's no need for this else if. It was causing problems with the automatic `<body>` extraction, since it was impossible to populate `obj.contents` without a fragment specified.