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

[YunchuWang]

Part of change to address Azure/azure-functions-python-library#107
The lib change has to go first.

TODOs:

1. Add unit tests to test new parsing and tonullable methods created.
2. Add asgi and wsgi tests
3. Add pystein tests

Fixes #

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