Fix Gitlab Milestone, Author field type

[PhompAng]

Gitlab Milestone

Refer to #322

• Gitlab Milestone due_date and start_date only send a date.

Gitlab Author

• Author email can be null if the user doesn't set a public email address.

[f-meloni]

It would be nice to add a unit test for this case

[Vacxe]

Have you tried to use default ISO8601 serializer?

Example

...
@Serializable(with = LocalDateIso8601Serializer::class) 
@SerialName("due_date")
val dueDate: LocalDate? = null
....

[PhompAng]

Thank you for the comment. I've updated the unit test and use the default serializer.