UP-4177 When attribute to duplicate is missing, no-op.

Make `AttributeDuplicatingPersonAttributesScript` that was introduced in #408 ([UP-4177](https://issues.jasig.org/browse/UP-4177)) no-op when the key-to-duplicate is not found.  Also refactor looping and enrichen unit test.

 * Make the script return the existing attributes map un-changed when the key to duplicate is not present in the existing attributes map.
 * Use idiomatic forEach in groovy ( `for (desiredName in desiredNames) {` )
 * use `assertEquals` with message rather than bare Groovy `assert` keyword in unit test
 * test the full content of the resulting attribute map against expected beyond just testing the resulting size.