Use get() instead of accessing by key ('ga_exp')

[adinhodovic]

No description provided.

[codecov]

Codecov Report

Merging #40 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #40   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          79     79           
  Branches       12     12           
=====================================
  Hits           79     79

Flag	Coverage Δ
#unittests	100% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
django_google_optimize/utils.py	100% <ø> (ø)	⬆️

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