Fix conversation context (and add tests)

[aoberoi]

Summary

This change fixes an issue where events that did not have a conversation context would get "stuck" instead of flowing through the middleware and listeners.

But more significantly, this change provides an implementation of unit testing middleware. This pattern have be used to test other middleware as well. This involved pulling common test helpers out of the *.spec.ts files, and placing them into test-helpers.ts.

Fixes #185

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 #187 into master will increase coverage by 4.53%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #187      +/-   ##
==========================================
+ Coverage   54.42%   58.95%   +4.53%     
==========================================
  Files           7        7              
  Lines         441      441              
  Branches      120      120              
==========================================
+ Hits          240      260      +20     
+ Misses        193      173      -20     
  Partials        8        8

Impacted Files	Coverage Δ
src/helpers.ts	100% <ø> (+5%)	⬆️
src/ExpressReceiver.ts	31.39% <ø> (ø)	⬆️
src/conversation-store.ts	100% <100%> (+76%)	⬆️

---

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 3080b5f...10263e2. Read the comment docs.