chore(headers): let overwrite headers

[jssevestre]

Invert dict to have the ability to overwrite default headers
in our case : user-agent should be lumapps-toolbox/{version}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fe1889d) 85.64% compared to head (3676869) 85.64%.
Report is 1 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #194   +/-   ##
=======================================
  Coverage   85.64%   85.64%           
=======================================
  Files          16       16           
  Lines         843      843           
  Branches      168      168           
=======================================
  Hits          722      722           
  Misses         78       78           
  Partials       43       43           

Flag	Coverage Δ
unittests	85.64% <ø> (ø)

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Files	Coverage Δ
lumapps/api/base_client.py	79.21% <ø> (ø)