Webhook & Message Stream bugs

[tgrosinger]

I found these bugs in my previous implementations as I started using these endpoints.

[codecov-commenter]

Codecov Report

Merging #36 (79a1820) into master (5ed9b01) will decrease coverage by 0.49%.
The diff coverage is 40.00%.

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@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   96.61%   96.13%   -0.49%     
==========================================
  Files          13       13              
  Lines         591      595       +4     
==========================================
+ Hits          571      572       +1     
- Misses         14       16       +2     
- Partials        6        7       +1     

Flag	Coverage Δ
unittests	96.13% <40.00%> (-0.49%)	⬇️

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Files Changed	Coverage Δ
message_streams.go	96.61% <ø> (ø)
webhooks.go	94.23% <40.00%> (-5.77%)	⬇️

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[mrz1836]

LGTM