Trim the response headers with a backward scan

[calixteman]

Removing the trailing whitespace with a $-anchored regex is quadratic in the length of the run, which a server controls. The helper lives in network_utils.js, to be unit testable.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.01%. Comparing base (7fc7072) to head (cba911d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21684   +/-   ##
=======================================
  Coverage   90.00%   90.01%           
=======================================
  Files         263      263           
  Lines       66854    66858    +4     
=======================================
+ Hits        60173    60180    +7     
+ Misses       6681     6678    -3     

Flag	Coverage Δ
browsertest	66.55% <0.00%> (+0.01%)	⬆️
fonttest	9.04% <ø> (ø)
integrationtest	69.36% <100.00%> (+<0.01%)	⬆️
unittest	57.83% <100.00%> (-0.01%)	⬇️
unittestcli	56.56% <100.00%> (+<0.01%)	⬆️

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[timvandermeij]

Looks good; thank you!