Fix invalid redirect computation (fix #285)

[michael-doubez]

Change redirect computation logic to accomodate for jenkins configured with path prefix.
This fixes the regression introduced in #261 while keeping the check on open redirect vulnerability.

Testing done

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6. Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.61%. Comparing base (22331f0) to head (37c11e7).
Report is 6 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #286      +/-   ##
============================================
+ Coverage     71.90%   72.61%   +0.70%     
+ Complexity      194      187       -7     
============================================
  Files             9        9              
  Lines           744      745       +1     
  Branches        124      116       -8     
============================================
+ Hits            535      541       +6     
+ Misses          150      146       -4     
+ Partials         59       58       -1     

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[michael-doubez]

Change validated by hand as well