Move ResponseWriter wrapper to internal

[ribice]

Not sure if httputils is the best name.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.82%. Comparing base (24ffea5) to head (37b9d72).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #880      +/-   ##
==========================================
+ Coverage   82.79%   82.82%   +0.02%     
==========================================
  Files          55       55              
  Lines        4610     4622      +12     
==========================================
+ Hits         3817     3828      +11     
- Misses        636      637       +1     
  Partials      157      157              

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