use existing response.vary property to set vary header

[davidism]

closes #2345

[codecov]

Codecov Report

Merging #2346 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2346      +/-   ##
==========================================
- Coverage   92.86%   92.85%   -0.02%     
==========================================
  Files          46       46              
  Lines        6423     6414       -9     
  Branches      503      501       -2     
==========================================
- Hits         5965     5956       -9     
  Misses        336      336              
  Partials      122      122

Impacted Files	Coverage Δ
flask/helpers.py	82.77% <ø> (-0.41%)	⬇️
flask/sessions.py	95.7% <100%> (-0.03%)	⬇️
tests/test_basic.py	96.5% <100%> (ø)	⬆️

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