Respond 400 instead of 500 when first header field line starts with SP or HTAB

[kenballus]

❓ What kind of change does this PR introduce?

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)
#728

❓ What is the current behavior? (You can also link to an open issue here)
Cheroot responds 500 when it receives a request in which the first header field line starts with SP or HTAB, due to an UnboundLocalError.

❓ What is the new behavior (if this is a feature change)?
It responds 400 instead.

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• I wrote descriptive pull request text above
• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after
  the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.67%. Comparing base (1ff20b1) to head (3fa1439).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #729      +/-   ##
==========================================
+ Coverage   83.65%   83.67%   +0.02%     
==========================================
  Files          28       28              
  Lines        4166     4178      +12     
==========================================
+ Hits         3485     3496      +11     
- Misses        681      682       +1