Import Contacts: Contact Matching on External Identifie for Fill mode is broken

(Originally reported by farmerchyrs on the forum - http://forum.civicrm.org/index.php/topic,6518.0/topicseen.html - , and replicated by me on latest 2.2)
--------------------
Attempt to use contact matching on "External Identifier" field fails with this error when "For Duplicate Contacts" (onDuplicate) is set to "Fill":
"External Identifier already exists in database."

The same configuration / steps works properly if onDuplicate is "Update".

To recreate with attached sample file...
1. Set Strict contact matching rules for Individuals, Orgs and Households to "External Identifier" ONLY (no other fields)

2. Import the sample file w/ "Skip" for duplicate contacts.

• Map fields = First, Last name, Street Address, State, External Identifier
  (do NOT map the Email field on this pass)
• Tag the import records so you can find them easily. You should have 5 records imported.

3. Repeat the import with the same file

• Use "Fill" for onDuplicate
• Map fields = Email, External Identifier ONLY

After fixing the problem..

• Verify that the fix works with Individual, Org and Household imports.
• Verify that the same problem is NOT present when using the default strict rule ("Email").