Fix issue where content-type headers were not sent to event functions.

[taeold]

This issue only affected python functions because Flask by default will return status=415 when making POST request with JSON body without content-type header. Express server wrapping Nodejs functions, on the other hand, is more loosey-goosey when it comes to validating incoming request headers.

Fixes #7998

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.18%. Comparing base (49c5ac5) to head (1db1a61).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8019      +/-   ##
==========================================
- Coverage   51.19%   51.18%   -0.01%     
==========================================
  Files         423      423              
  Lines       29544    29547       +3     
  Branches     6031     6032       +1     
==========================================
  Hits        15125    15125              
- Misses      13059    13062       +3     
  Partials     1360     1360              

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