listPullRequests({ state: 'all' }) does not return all pull requests.

# Reproduction

```js
const assert = require('assert')
const Github = require('github-api')

const github = new Github()

;(async () => {
const repo = github.getRepo('vuejs', 'rfcs')

const all = (await repo.listPullRequests({ state: 'all' })).data.length
const open = (await repo.listPullRequests({ state: 'open' })).data.length
const closed = (await repo.listPullRequests({ state: 'closed' })).data.length

assert.equal(all, open + closed)
})()
```

# See

https://github.com/vuejs/rfcs/pulls