Fixed bug from calling $logout() multiple times

[jwngr]

@katowulf - This fixes #431. We used to set this._currentUserData to undefined every time $logout() was called. This worked in most cases since the called to this._authClient.logout() ended up firing out _onLoginEvent() callback which set this._currentUserData to null and resolved any leftover getCurrentUser() promises. However, if we called this._authClient.logout() when we were already logged out, Simple Login does not fire the _onLoginEvent() callback and this._currentUserData gets stuck as undefined, which causes problems for logging in and getting the current user. Having this._currentUserData as undefined should only be valid at the very initializing of this service. It should always otherwise be null or a valid user object. As a result, this change just sets it to null when we call $logout().

cc/ @davideast

[katowulf]

Great detective work! Looks ☃

[coveralls]

Coverage remained the same when pulling 766495b on jw-logout-bug into 9eeaa5c on master.