Bump supertest from 6.1.5 to 6.1.6

Bumps [supertest](https://github.com/visionmedia/supertest) from 6.1.5 to 6.1.6.
- [Release notes](https://github.com/visionmedia/supertest/releases)
- [Commits](forwardemail/supertest@v6.1.5...v6.1.6)

---
updated-dependencies:
- dependency-name: supertest
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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Author: dependabot[bot]