Improve import update address fields granularity (currently updating one address field wipes all others)

This is a bug or a feauture request depending on your POV.

I used import in update mode to update one field of address data (country). After doing the import, when I look at the record, the entire address block has been overwritten so for example:

"Chesil Cottage, Moushill Lane, Milford, Surrey, United Kingdom"

becomes

"United Kingdom".

I would err on the side that this is a bug unless it is explicity documented because the import screen says "Update: Updates database fields with available import data. Fields in the database which are NOT included in the import row are left as-is."