Edit metadata API: doesn't add or replace if the value "contains" the current value

Using the edit metadata API, I tried both adding and replacing an author (note, in practice it is more likely useful as a "replace"):

Current value: Messi, Lionel
New Value: Messi, Lionel A.

Nothing happened - this is because the check uses contains logic (instead of equality) to check the metadata. This is because it is comparing to the display value which concatenates multiple fields, so we'll need to figure out a clever way to replace / fix this logic.

Start at the edit metadata API entry point and trace how the current metadata is converted to a display value before the contains check. Reproduce both add and replace cases using an existing value that contains the new value. Done means the API distinguishes complete metadata values and correctly adds or replaces the author without false matches.