SMACK-459 actually add the defaultIdentity to the result of getIdentities()

git-svn-id: http://svn.igniterealtime.org/svn/repos/smack/trunk@13796 b35dd754-fafc-0310-a699-88a17e54d16e
SMACK-459 Make it user proof by returning an unmodifiable set again

git-svn-id: http://svn.igniterealtime.org/svn/repos/smack/trunk@13797 b35dd754-fafc-0310-a699-88a17e54d16e