Allows WebClient token to be undefined

[aoberoi]

Summary

this PR also introduces a new set of integration tests that are useful for running the typescript compiler as if a consumer of this package were to using it.

fixes #482

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 #499 into master will increase coverage by 1.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #499      +/-   ##
==========================================
+ Coverage   80.08%   81.32%   +1.24%     
==========================================
  Files           6        6              
  Lines         241      241              
  Branches       37       37              
==========================================
+ Hits          193      196       +3     
+ Misses         30       29       -1     
+ Partials       18       16       -2

Impacted Files	Coverage Δ
src/WebClient.ts	80.91% <ø> (+2.29%)	⬆️

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