[#75]: Add missing draft field to HookPullRequest

[dermetfan]

Issue reference:

I didn't open an issue as this seemed simple enough.

Submission Checklist:

• Have you followed the guidelines in our Contributing document (for example, is your tree a clean merge)?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Does your submission build?
• Does your submission pass tests?
• Have you run lints on your code locally prior to submission?
• Have you updated all of the cabal/nix infrastructure?
• Is this a breaking change? Have you discussed this?
  • → Is it? I'm very new to Haskell. I suppose it does break instantiations so that they now result in a missing field error.

[kvanbere]

Thanks @dermetfan . LGTM

Hey @roberth , are you in a position to test if this breaks any of your stuff?

[kvanbere]

Hm, let me schedule it for testing here and we'll get back to you soon. Sorry about the delay.

[kvanbere]

Looks good to me, sorry that took so long. :)

[kvanbere]

@dermetfan could I ask you to please open a second PR updating the changelog with this change? It is a breaking change (number of fields in record), but we forgot to add it.