Allow for updating individual fields without blanking remaining fields

If the GUID matches on an import, it would be nice if the import just updated the fields with data present, leaving the others intact rather than blanking them. This would reduce the record size on updates being sent from another source.

For the use case I'm working with, there's an external database that tracks widgets as they go through a process. After the initial feed node creation, it would be nice just to send GUID and updated fields (as CSV, with header). Right now, doing that blanks all of the other (previously imported) fields. This is particularly important for me because the 3rd party database that produces the feed does not preserve historical data that I want to preserve in the drupal node.