What this diff mean is that if a object with a member type changed from T to Option[T],  after this object generates a JValue object according to its data members, and then this JValue is rendered as a json string, it may have a chance that this field do not have a stringified output. So after this string is parsed again as a JValue object, there will be a diff between this two json. 
This is for testing someone change a data member type from T to Option[T],  and not assign it when jsonize. This test will find this problem and assert fail. This is what I think what @Partick means in that JIRA. @rxin Any good idea for this change?