feat(Request): add Request.headers_lower to provide a normalized interface between WSGI and ASGI

[kgriffs]

Summary of Changes

This patch adds a headers_lower property to provide a unified way to get all headers lowercased for middleware, etc. that needs to be compatible with both WSGI and ASGI. This was done in lieu of introducing a breaking change to the WSGI headers property.

Related Issues

N/A

Pull Request Checklist

This is just a reminder about the most common mistakes. Please make sure that you tick all appropriate boxes. But please read our contribution guide at least once; it will save you a few review cycles!

If an item doesn't apply to your pull request, check it anyway to make it apparent that there's nothing to do.

• [ ] Applied changes to both WSGI and ASGI code paths and interfaces (where applicable).
• [ ] Added tests for changed code.
• [ ] Prefixed code comments with GitHub nick and an appropriate prefix.
• [ ] Coding style is consistent with the rest of the framework.
• [ ] Updated documentation for changed code.
  • [ ] Added docstrings for any new classes, functions, or modules.
  • [ ] Updated docstrings for any modifications to existing code.
  • [ ] Updated both WSGI and ASGI docs (where applicable).
  • [ ] Added references to new classes, functions, or modules to the relevant RST file under docs/.
  • [ ] Updated all relevant supporting documentation files under docs/.
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PR template inspired by the attrs project.

[codecov[bot]]

Codecov Report

Merging #2063 (f6f6c4a) into master (47e3e16) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2063   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           63        63           
  Lines         6715      6722    +7     
  Branches      1242      1244    +2     
=========================================
+ Hits          6715      6722    +7     

Impacted Files	Coverage Δ
falcon/asgi/request.py	100.00% <100.00%> (ø)
falcon/request.py	100.00% <100.00%> (ø)

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[CaselIT]

@kgriffs you forgot to commit the news fragment