Refactor client

[tpazderka]

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

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Touch of refactoring...

Removed duplicated methods on Clients.

[codecov-io]

Codecov Report

Merging #626 into master will increase coverage by 0.28%.
The diff coverage is 70.58%.

@@            Coverage Diff             @@
##           master     #626      +/-   ##
==========================================
+ Coverage   61.51%   61.79%   +0.28%     
==========================================
  Files          62       62              
  Lines       11140    11094      -46     
  Branches     1973     1956      -17     
==========================================
+ Hits         6853     6856       +3     
+ Misses       3727     3681      -46     
+ Partials      560      557       -3

Impacted Files	Coverage Δ
src/oic/oic/__init__.py	66.13% <100%> (+0.7%)	⬆️
src/oic/oauth2/__init__.py	69.33% <50%> (+3.83%)	⬆️
src/oic/utils/http_util.py	68.67% <0%> (+0.57%)	⬆️
src/oic/utils/authn/user.py	64.22% <0%> (+0.81%)	⬆️

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[tpazderka]

Part of #629

[schlenk]

LGTM