Fix PAR request body encoding and add stricter platform guards

[radmakr]

I needed some of these changes for CoreATProtocol and I think they can safely be merged in here. I'm happy to keep this as a branch in my fork if you think these changes don't make sense.

[radmakr]

I just noticed #54 which on first look seems much more comprehensive than what I did here.

[mattmassicotte]

Thank you so much for taking the time to do this! You're right, there are a bunch of pending PRs that need my attention, and I've committed to looking at them first. However, once that's all done, let's revisit and see where we're at?

[radmakr]

looking through all of Emelia's PRs, my PR here probably isn't needed. Once her PRs are merged I'll test locally and then we can probably close this.

[mattmassicotte]

Ok, well let's leave it until we actually get to that point. And either way, I really appreciate you doing this!

[radmakr]

I can confirm that after the recent merged PRs, these changes are no longer required. I am closing the PR.

[mattmassicotte]

Well, sorry for the wasted work here. I still really appreciate you doing it!

[radmakr]

Not a waste. I learned a lot!