PXD-1089 ⁃ PUT new url appends to list

[macintoshpie]

I spoke with Phillis about this a while back, so I may be misremembering. But at the moment whenever you make a PUT request to update a record's urls attribute, the url is appended to a list. It seemed to me that we wanted to just update the previous url instead.

@philloooo please correct me if I am wrong/further elaborate if necessary.