SLING-2998 SlingAuthenticator fails because of pathInfo being null

- concatenate servlet path and path info into path to check against
- return anonymous credentials/false for empty path
- remove LoginServlet.SERVLET_PATH.equals(pathInfo) check

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1522946 13f79535-47bb-0310-9956-ffa450edef68