fix: http courier should use lower case json

[aeneasr]

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
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• I have added tests that prove my fix is effective or that my feature
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• I have added or changed the documentation.

Further Comments

[aeneasr]

@jonas-jonas the payloads are currently uppercased and do not comply with our API design guidelines. This is a breaking change, can you please check if someone is using this already? We would need to inform them of these changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fd79950) 78.27% compared to head (5335e2b) 78.30%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3740      +/-   ##
==========================================
+ Coverage   78.27%   78.30%   +0.03%     
==========================================
  Files         347      347              
  Lines       23813    23813              
==========================================
+ Hits        18639    18647       +8     
+ Misses       3762     3756       -6     
+ Partials     1412     1410       -2     

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