fixed ack in ExpressReceiver firing twice. Issue #327

[stevengill]

Summary

Issue #327. Thanks @jarrodldavis for the patch!

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 #370 into master will decrease coverage by 0.16%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #370      +/-   ##
==========================================
- Coverage    83.2%   83.03%   -0.17%     
==========================================
  Files           7        7              
  Lines         500      501       +1     
  Branches      146      146              
==========================================
  Hits          416      416              
- Misses         55       56       +1     
  Partials       29       29

Impacted Files	Coverage Δ
src/ExpressReceiver.ts	68.57% <0%> (-0.66%)	⬇️

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