always call wp_authenticate from AtomPub

[wnorris]

The included patch makes a small change to the AtomPub code which causes it to always attempt user authentication via wp_authenticate(). Previously, it was only making this call if the BasicAuth server parameters were set. In order for OAuth (or any alternate authentication mechanism) to work wp_authenticate() must always be called in order to give them a chance to authenticate the request.

[ryan]

(In [10771]) Always wp_authenticate in wp-app. Props wnorris. fixes #9320