Pull requests are not included when using "--data-source=milestones"

In github-releases-notes v0.16.0, when using --data-source=milestones with "gren changelog --generate", only the issues closed by that milestone are included.

Sample GitHub.com repository milestone: https://github.com/nanocurrency/raiblocks/milestone/11?closed=1

Command: gren changelog --generate --username nanocurrency --repo raiblocks --tags V16.1 --changelog-filename ./CHANGELOG.md --override --milestone-match '{{tag_name}}' --ignore-tags-with="RC,^VERSION" --data-source milestones`

Actual Output:
```
# Changelog

## V16.1 (26/09/2018)
- [**bug**] Mutex deadlock [#1235](https://github.com/nanocurrency/raiblocks/issues/1235)
- [**bug**] Outbound Usage [#1214](https://github.com/nanocurrency/raiblocks/issues/1214)
- [**bug**] Increase stack size for Windows builds [#1212](https://github.com/nanocurrency/raiblocks/issues/1212)
- [**bug**] Rai node very slow sync since v15 [#1204](https://github.com/nanocurrency/raiblocks/issues/1204)
```

Expected output:
In addition to the 4 issues, all the merged in pull requests should be included

首先，使用报告中的命令和仓库示例，通过 `gren changelog --generate` 和 `--data-source=milestones` 复现该问题。跟踪里程碑数据的收集过程，并将包含的 issue 与已合并的 pull request 进行比较；当生成的 changelog 包含该里程碑的这两者时，即表示完成。

由索引模型根据 Issue 内容生成。