fix custom HTTPException with argument names

[aniketmaurya]

Before submitting

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Fixes #301 : custom Exception HTTPException(detail="...", status_code=400) doesn't work. This PR fixes that.

• fastapi.HTTPException is not pickleable, we add a __reduce__ method and put the correct exception in the multiprocessing queue.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95%. Comparing base (e8ffd36) to head (894ed6a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #302   +/-   ##
===================================
  Coverage    95%    95%           
===================================
  Files        19     19           
  Lines      1237   1257   +20     
===================================
+ Hits       1170   1195   +25     
+ Misses       67     62    -5