RetainOperator retains the first occurrence of a field, without changing the header.
RetainOperator is intended for fields where the value is expected to be the same for all occurrences of the unique key, and the goal is to pass the value through unchanged. It is like FirstOperator, except that the original header is preserved. The original header preservation is setup in the call to the SingleFieldOperation constructor.
Notes: - An option to signal an error if multiple values are encountered might be useful.