DirectoryScanner.isSymbolicLink bug: test parent and not the complete file (Java 7 and more)

Following #27, [DirectoryScanner.isSymbolicLink](https://github.com/codehaus-plexus/plexus-utils/blob/master/src/main/java/org/codehaus/plexus/util/DirectoryScanner.java#L732) seems bugged: only parent tested, not the complete file (for Java 7 and more).

PR : #29
