Fix for GH#107: Enable support for multiple Set-Cookie response headers (native & wsgi/asgi)

[YunchuWang]

Part of change to address #107
Related change in worker: Azure/azure-functions-python-worker#1004

Changes

1. Make HttpResponseHeader inherit from werkzeug header class which is also used in Flask, the header class has dict-like interface but is ordered and can store the same keys multiple times, all operations are case insensitive on header key.
2. Cookie headers are extracted from httpresponseheaders and sent to worker as cookie field in rpchttp message.

[codecov]

Codecov Report

Merging #115 (b99ed62) into dev (990bb95) will increase coverage by 0.61%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #115      +/-   ##
==========================================
+ Coverage   85.00%   85.61%   +0.61%     
==========================================
  Files          47       47              
  Lines        2774     2781       +7     
  Branches      371      374       +3     
==========================================
+ Hits         2358     2381      +23     
+ Misses        341      324      -17     
- Partials       75       76       +1     

Flag	Coverage Δ
unittests	85.61% <100.00%> (+0.61%)	⬆️

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

Impacted Files	Coverage Δ
azure/functions/_abc.py	75.82% <100.00%> (+0.08%)	⬆️
azure/functions/_http.py	91.30% <100.00%> (+6.95%)	⬆️
azure/functions/http.py	51.38% <100.00%> (+16.54%)	⬆️

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[vrdmr]

LGTM.

Left some comments and we can discuss offline to go over the details.