PLXCOMP-220: fixed regression with attribute merging

Found a silly mistake that I introduced in refactoring resource collections, which I fixed. But having looked at the previous implementations of the two resource collection implementations (file and proxy), I realised that file resource collection was using FileAttributes, which had a slightly different way of handling file/directory modes. Effectively, FileAttributes would remove any file mode bits that were not part of 0777 mask. Therefore, to stay on the safe side, I decided to push down the implementation from the abstract class and restore the initial behaviour.

All in all, we must be having a correct uid/gid merging behaviour now, and all recently introduced regressions must have been fixed on this branch.

I have tried running plexus-archiver tests against the latest snapshot of plexus-io, and all tests pass now.