🐛 Fix bug overriding custom HTTPException and RequestValidationError from exception_handlers

[uriyyo]

Fix issue/related to #1440.

A fix is simple, set HTTPException and RequestValidationError exception handlers only in case when they don't set.

[codecov]

Codecov Report

Merging #1924 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1924   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          239       240    +1     
  Lines         7079      7102   +23     
=========================================
+ Hits          7079      7102   +23     

Impacted Files	Coverage Δ
fastapi/applications.py	100.00% <100.00%> (ø)
tests/test_exception_handlers.py	100.00% <100.00%> (ø)

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[github-actions]

📝 Docs preview for commit 391fba2 at: https://5f3d6c9962cbd41e7c6cc084--fastapi.netlify.app

[uriyyo]

This PR solve same issue as #1887 solve, I didn't see it because they point to two different issues related to the same problem.

Basically my idea was to reduce the number of calls to Starlette.build_middleware_stack method.

Previously there can be 3 calls to this method because in FastAPI.setup will call FastAPI.add_exception_handler method which will call Starlette.build_middleware_stack.

With this bugfix, there will be always only one call to the Starlette.build_middleware_stack method.

[tiangolo]

Awesome! Great job @uriyyo ! ☕ 🍰

Thanks for pointing to the previous PR. I ended up merging this one as the code was slightly simpler 🤓

This is included in FastAPI 0.61.2 (released in some minutes).