GH#107: Skip setting multi Set-Cookie headers for python 3.7

[YunchuWang]

Skipping setting multiple Set-Cookie headers as SimpleCookie which is a standard python lib api in python3.7 does not support samesite attribute and would cause cookie parsing error. (Ref: https://docs.python.org/3/library/http.cookies.html). A warning will be given to customers who uses python3.7 and tries to set multiple cookie headers.

[codecov]

Codecov Report

Merging #125 (7548d66) into dev (6f10a44) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #125      +/-   ##
==========================================
+ Coverage   85.85%   85.87%   +0.01%     
==========================================
  Files          49       49              
  Lines        2863     2867       +4     
  Branches      388      389       +1     
==========================================
+ Hits         2458     2462       +4     
  Misses        329      329              
  Partials       76       76              

Flag	Coverage Δ
unittests	85.83% <85.71%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
azure/functions/http.py	53.94% <100.00%> (+2.55%)	⬆️

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