Fix arg for WSGI start_response

[tpazderka]

Close #515

• Any changes relevant to users are recorded in the CHANGELOG.md.
• The documentation has been updated, if necessary.

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[codecov-io]

Codecov Report

Merging #516 into release.0.13.0 will increase coverage by <.01%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           release.0.13.0     #516      +/-   ##
==================================================
+ Coverage           59.04%   59.04%   +<.01%     
==================================================
  Files                  62       62              
  Lines               11214    11216       +2     
  Branches             1959     1959              
==================================================
+ Hits                 6621     6623       +2     
  Misses               4036     4036              
  Partials              557      557

Impacted Files	Coverage Δ
src/oic/utils/http_util.py	66.04% <100%> (+0.18%)	⬆️

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[schlenk]

LGTM.