Support for the Unpoly 2 protocol

[triskweline]

Thank you for maintaining this package!

This is just a heads up that Unpoly 2 is going to be released this week.

Unpoly 2 comes with several extensions to the optional server protocol that this repo implements. All protocol changes are backwards compatible, so Unpoly 2 users can keep using this package.

You may want to consider whether you want to implement some of the newer features like X-Up-Events, X-Up-Mode or X-Up-Accept-Layer.

If you're planning to stick with the current feature set, that's cool too, of course.

[robinvdvleuten]

@triskweline quite some time passed when I looked at this package 😅 Is see that Unpoly is at version 2.7. Is only implementing the newer features you mentioned still sufficient to support 2.x?

[triskweline]

@robinvdvleuten No new headers were added to the protocol during 2.x. Also all headers are optional, so you're free to pick and choose what you find the most important, or what's convenient to implement.

On a related note, Unpoly 3 is going to be released in some weeks. Once we're ready I'm going to open an issue like this with suggestions for the most relevant changes.

Thanks for maintaining this.