Case insensitive decorator remove() and containsKey() behavior difference.

[cheqianh]

Issue was raised https://github.com/amzn/ion-hive-serde/pull/59#discussion_r733971888

In a case insensitive decorator, containsKey(fieldName) returns true once it finds fieldName ignoring cases while remove(fieldName) is not.

So it's possible struct.containsKey("foo") == true while struct.remove("foo") does nothing.