Fix SARIMAXModel saving init params

[Mr-Geekman]

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[codecov-commenter]

Codecov Report

Merging #81 (487f3a7) into master (d152d9a) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   90.14%   90.23%   +0.08%     
==========================================
  Files          48       48              
  Lines        2152     2170      +18     
==========================================
+ Hits         1940     1958      +18     
  Misses        212      212              

Impacted Files	Coverage Δ
etna/models/sarimax.py	93.87% <100.00%> (+1.37%)	⬆️

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