Updating only changed fields in a standard controller

UpdateChangedFieldsController.cls

public class UpdateChangedFieldsController {
	SObject oldRecord, currentRecord;
	public UpdateChangedFieldsController(ApexPages.StandardController controller) {
		oldRecord = controller.getRecord().clone();
		currentRecord = controller.getRecord();
	}
	public PageReference saveChanges() {
		SObject newClone = currentRecord.getSObjectType().newSObject(currentRecord.Id);
		Map<String, Object> 
			oldValues = oldRecord.getPopulatedFieldsAsMap(),
			newValues = currentRecord.getPopulatedFieldsAsMap();
		for(String key: newValues.keySet()) {
			if(newValues.get(key) != oldValues.get(key)) {
				newClone.put(key, newValues.get(key));
			}
		}
		try {
			upsert newClone;
			return new ApexPages.StandardController(newClone).view();
		} catch(Exception e) {
			return null;
		}
	}
}