Fix docs stating that body is the default value for fragment

That's not exactly true, as we require users to specify an explicit
`fragment` value if they're dealing with full-HTML responses from the
server. This explicit value can be "body" if the user chooses so, but
that must be decided on a per-app basis.

If no explicit `fragment` value is provided, a full-HTML response will
trigger a native reload of the page.

Closes #475 [ci skip]

Author: mislav

README.md

@@ -163,7 +163,7 @@ key | default | description
 `container` | | CSS selector for the element where content should be replaced
 `url` | link.href | a string or function that returns the URL for the ajax request
 `target` | link | eventually the `relatedTarget` value for [pjax events](#events)
-`fragment` | `"body"` | CSS selector for the fragment to extract from ajax response
+`fragment` | | CSS selector for the fragment to extract from ajax response
 
 You can change the defaults globally by writing to the `$.pjax.defaults` object: