contact merge: communication preferences broken

when merging two contacts, communication preferences are not handled correctly.
patch attached.

Deepak – you should review. there are two issues –
we revert to the original array instead of the concatenated list of preferences constructed earlier when we pass the value. and secondly, the use of CRM_Core_OptionGroup::lookupValues strips the leading/trailing VALUE_SEPARATOR, so we need to re-add it. it probably makes sense to just remove the inclusion of those earlier so that it only is added once.