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

Start by reproducing the issue with `gren changelog --generate` and `--data-source=milestones` using the command and repository example in the report. Trace how milestone data is collected and compare the included issues with merged pull requests; done means the generated changelog includes both for the milestone.

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