Make context.issue and context.repo objects

[jclem]

The way that context.issue and context.repo are functions that merge their arguments has been a point of confusion for me quite a bit.

Given that Node supports object spread, and you can achieve the same goal with that, does it make sense for context.issue and context.repo to just be (frozen) objects, instead? Before I read over the readme in much more detail, it's how I expected it to work, and looks a little more idiomatic to me.

const payload = context.issue({body: 'Hello, world!'})

vs

const payload = {...context.issue, body: 'Hello, world!'}

[JasonEtco]

I'm so glad that someone finally said something 😅 this is a remnant of Probot's code (I quite literally copy-pasted), and that decision was made in Probot before the object spread operator was in LTS Node. That's not a good enough reason for it to be here though!

Unfortunately, this is the kind of change that will break almost every action, so it'll need mean a breaking change/new major version to v2 (which should already be done for #41). In any case, still very much in favor.

[jclem]

Sounds good! This was a quick change, so no worries if you close this PR and implement it later!

[JasonEtco]

I think this is a good enough reason to introduce a new major version, especially because of #41 - I'll get a v2 branch going and update this PR's base 🎉 Thanks @jclem ❤️