Sarimax bug in future prediction with quantiles

[martins0n]

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Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Proposed Changes

Related Issue

Closing issues

closes #375

[codecov-commenter]

Codecov Report

Merging #391 (9cfd839) into master (08b0f81) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #391   +/-   ##
=======================================
  Coverage   87.02%   87.02%           
=======================================
  Files          98       98           
  Lines        4964     4964           
=======================================
  Hits         4320     4320           
  Misses        644      644           

Impacted Files	Coverage Δ
etna/models/sarimax.py	96.00% <100.00%> (ø)

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