Case Insensitive headers test

[kochniev]

Fixes CSL3-2108

Description of changes in release / Impact of release:

Adds a Python test to cover the case-insensitive headers use case

Is this a breaking change?

• [ ] Yes
• [x] No

If you answered Yes then describe why is it so

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Is there a way to disable the change?

• [x] Use previous release
• [ ] Use a feature flag
• [ ] No

Additional details go here

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