[v2] Bring PR #417 changes to v2 branch

[seratch]

Summary

This pull request brings the changes by #417 (thanks @greggTime !) to the v2 branch. It had some conflicts so that I came up with a plain commit.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #418 into @slack/bolt@next will not change coverage.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           @slack/bolt@next     #418   +/-   ##
=================================================
  Coverage             85.51%   85.51%           
=================================================
  Files                     7        7           
  Lines                   511      511           
  Branches                146      146           
=================================================
  Hits                    437      437           
  Misses                   49       49           
  Partials                 25       25

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1428ec3...8aa5727. Read the comment docs.