Fix #705 by modifying SlackResponse to consider None value in dict

[seratch]

Summary

This pull request fixes #705 by modifying SlackRepsonse to consider possible None values in req_args dict.

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.

[seratch]

@stevengill @aoberoi
I will be releasing a new patch version including this change tomorrow. Let me know if you have comments or feedback by then 🙇

[codecov]

Codecov Report

Merging #706 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #706      +/-   ##
==========================================
+ Coverage   85.66%   85.68%   +0.02%     
==========================================
  Files          19       19              
  Lines        2630     2634       +4     
  Branches      251      252       +1     
==========================================
+ Hits         2253     2257       +4     
  Misses        282      282              
  Partials       95       95              

Impacted Files	Coverage Δ
slack/web/slack_response.py	98.14% <100.00%> (+0.14%)	⬆️

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