Only add HSTS headers in HTTPS

[aledbf]

Which issue this PR fixes:

fixes #2073

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf

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[k8s-reviewable]

This change is 

[codecov-io]

Codecov Report

Merging #2143 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2143   +/-   ##
=======================================
  Coverage   36.53%   36.53%           
=======================================
  Files          68       68           
  Lines        4845     4845           
=======================================
  Hits         1770     1770           
  Misses       2807     2807           
  Partials      268      268

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