Fix #377 ExpressReceiver's RespondFn implementation doesn't accept a string

[seratch]

Summary

This pull request fixes #377 by updating the response function generation in ExpressReceiver. I wanted to write some tests for it but it's still not so easy to have uni tests for the internals of the requestHandler method. So, I just cut and pasted the same logic to a test suite this time.

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 #379 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
+ Coverage   82.86%   83.13%   +0.26%     
==========================================
  Files           7        7              
  Lines         502      510       +8     
  Branches      147      148       +1     
==========================================
+ Hits          416      424       +8     
  Misses         57       57              
  Partials       29       29

Impacted Files	Coverage Δ
src/ExpressReceiver.ts	70.17% <100%> (+2.25%)	⬆️

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