Fix incoming webhook agent test

[aoberoi]

Summary

It turned out that the tests committed in #640 didn't actually test anything. The first tests conveniently had the assertion that mattered commented out, and the second test never returned the Promise, so the unhandled rejection was going unnoticed in our CI for quite some time.

The feature they meant to test does actually work. I only added one line to the implementation, and that was for clarity, not because the tests wouldn't pass without it.

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 #719 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #719      +/-   ##
==========================================
+ Coverage   93.94%   93.95%   +0.01%     
==========================================
  Files           7        7              
  Lines         446      447       +1     
  Branches       43       43              
==========================================
+ Hits          419      420       +1     
  Misses         25       25              
  Partials        2        2

Impacted Files	Coverage Δ
src/IncomingWebhook.ts	92.85% <100%> (+0.26%)	⬆️

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