Fixed and refactored get_client_id

[tpazderka]

Close #445

• 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 #453 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #453     +/-   ##
=========================================
+ Coverage   58.21%   58.42%   +0.2%     
=========================================
  Files          61       61             
  Lines       11227    11218      -9     
  Branches     1908     1908             
=========================================
+ Hits         6536     6554     +18     
+ Misses       4147     4125     -22     
+ Partials      544      539      -5

Impacted Files	Coverage Δ
src/oic/utils/authn/client.py	68.11% <100%> (+6.81%)	⬆️

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