Mapping: preserve order of principal names

while using the new service mapping definition with principal names, I noticed that it might be better to preserve the order of the principal names as defined in the mapping. In particular if the repository login picks one of the configured names for the the javax.jcr.Session.getUserID().

Therefore I would like to propose to slightly change the implementation of Mapping.extractPrincipalNames to use a LinkedHashSet.

kpauls, would you mind reviewing the atttached patch and apply it? Thanks a lot!

cc: chaotic