Make all properties data classes

[ArnyminerZ]

Changes

• Got rid of HrefListProperty$toString: should be handled by the data class.
• Got rid of GetETag$toString: should be handled by the data class.
• Got rid of ResourceType$toString: should be handled by the data class.
• Moved HrefListProperty.hrefs to the constructor and made open so all the child classes can override it.
• Improved formatting of some files.

FIXME

• ScheduleTag has a custom toString method, do we want that implementation or use the data class one?
• I've made all the classes use val instead of var. I think it's the best approach, since the classes shouldn't be mutable, they represent an state given by the server, and it should be kept like that. We are only modifying the values for constructing the class, and I think it's not correct to keep them as vars.
• All HrefListProperty classes use LinkedList. Following the same principle as the point before, I think they should be immutable lists.

[rfc2822]

ScheduleTag has a custom toString method, do we want that implementation or use the data class one?

We can use the one from the data class.

I've made all the classes use val instead of var. I think it's the best approach, since the classes shouldn't be mutable, they represent an state given by the server, and it should be kept like that. We are only modifying the values for constructing the class, and I think it's not correct to keep them as vars. […]
All HrefListProperty classes use LinkedList. Following the same principle as the point before, I think they should be immutable lists.

Good 👍🏻

[rfc2822]

Nice :)