Fix creating CIMultiDict from MultiDict not making keys case-insentive

[bdraco]

When MultiDict was passed to CIMultiDict the keys need to be converted
fixes #1111

side note _multidict_extend doesn't appear to use name

[codspeed-hq]

CodSpeed Performance Report

Merging #1112 will not alter performance

_{Comparing convert_multidict_cimultidict (00ac3fa) with master (a9d54d7)}

Summary

✅ 244 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.27%. Comparing base (41cbc0d) to head (00ac3fa).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
multidict/_multidict_py.py	57.14%	0 Missing and 3 partials ⚠️

❌ Your project status has failed because the head coverage (83.58%) is below the target coverage (85.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1112      +/-   ##
==========================================
+ Coverage   91.22%   91.27%   +0.04%     
==========================================
  Files          42       42              
  Lines        6110     6176      +66     
  Branches      613      618       +5     
==========================================
+ Hits         5574     5637      +63     
  Misses        105      105              
- Partials      431      434       +3     

Flag	Coverage Δ
CI-GHA	91.27% <92.10%> (+0.04%)	⬆️
MyPy	83.58% <92.10%> (+0.08%)	⬆️
pytest	100.00% <ø> (ø)

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[bdraco]

Feel free to adjust this PR.

I'm not sure this regression is worth yanking 6.3.0 or if this this is even a real use case ?

[asvetlov]

All right, I think now the PR is good.