Update BlockAttachment to not send invalid JSON due to fields attribute

[paul-griffith]

Summary

Fixes a bug in #400; the base Attachment class that BlockAttachment inherits from will populate the fields attribute with an empty list, which then gets sent in the JSON to Slack. That makes the JSON invalid, meaning the BlockAttachment class as it is now cannot be used without manually adjusting the JSON/dict returned.

This PR updates the class, as well as adds a test that would have caught this.

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.

[paul-griffith]

I started using 2.1.0 internally and immediately ran into this 🤦‍♂️

[codecov]

Codecov Report

Merging #473 into master will increase coverage by 0.35%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #473      +/-   ##
==========================================
+ Coverage   69.58%   69.94%   +0.35%     
==========================================
  Files          15       15              
  Lines        1654     1657       +3     
  Branches       91       91              
==========================================
+ Hits         1151     1159       +8     
+ Misses        481      476       -5     
  Partials       22       22

Impacted Files	Coverage Δ
slack/web/classes/attachments.py	98.8% <100%> (+6.21%)	⬆️

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[RodneyU215]

Thanks for your diligence Paul!