Revert the removal of the workaround.

Unfurtunally de.hu_berlin.german.korpling.saltnpepper.salt.saltCore.accessors.SAnnotatableElementAccessor is still not Serializable.

since Salt version 1.1.8 the SDocument is Serializable and we don't need the workaround any longer (reverted from commit 173bc1d681f7a01b6ddb0eedefe24f385813c0c1)