Preserve oldest lead's source attribution fields in an auto-merge

When an older lead is auto-merged with a newer lead, the default behavior is for the newer lead's field values to overwrite the older lead's field values.  While this might be appropriate for many fields, it is rarely appropriate for source attribution fields like Original Referrer, Original Source Type, Original Search Engine, Original Search Phrase, Registration Source Type and Lead Source.  In addition, there may be custom attribution fields like Initial Online Source and Opt in Source whose older values should be preserved.  I suggest that at a minimum, standard attribution field values be preserved when merging with a newer lead.  Ideally, this behavior would be configurable for custom fields as well.