Add a workaround for OpenJDK 11's bug

[seratch]

Summary

Sending HTTP requests to Slack API server on OpenJDK 11 fails due to https://bugs.openjdk.java.net/browse/JDK-8211806

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 #452 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #452   +/-   ##
=========================================
  Coverage     83.70%   83.70%           
  Complexity     2387     2387           
=========================================
  Files           250      250           
  Lines          6438     6438           
  Branches        586      586           
=========================================
  Hits           5389     5389           
  Misses          690      690           
  Partials        359      359           

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