✅ Update response_model_by_alias

[tiangolo]

Update logic for response_model_by_alias to apply it only to the response_model, not to the object returned by the function.

And update tests with a small note in the tests clarifying (so I don't forget) that response_model_by_alias=False is basically just a quick hack that doesn't support OpenAPI.

The proper way to do it if the intention is to have OpenAPI support is to create another model (possibly inheriting from the original one).

This supersedes #1412

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[codecov]

Codecov Report

Merging #1642 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1642   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          236       237    +1     
  Lines         7068      7157   +89     
=========================================
+ Hits          7068      7157   +89     

Impacted Files	Coverage Δ
fastapi/routing.py	100.00% <ø> (ø)
tests/test_duplicate_models_openapi.py	100.00% <100.00%> (ø)
tests/test_response_by_alias.py	100.00% <100.00%> (ø)

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