Condition to not authenticate twice causes user to be not logged in during Create/Edit

[jkohlbach]

Hi, took me a while to narrow this down but it seems that the following code causes issues where users aren't logged in properly during Create/Edit calls:

// Don't authenticate twice
if ( ! empty( $user ) ) {
    return $user;
}

I comment this out and everything starts working properly.

I tested this on multiple single site installations on several servers. It was the only way to get the Basic Auth working.

I suspect the issue might have been introduced when some conditions for multi-site recursion was addressed? #9