Fix default extra response

[tiangolo]

Fix "default" extra response, when combined with responses with status codes and status code ranges.

Continuation of #435.

[codecov]

Codecov Report

Merging #489 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #489   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         244    244           
  Lines        5707   5708    +1     
=====================================
+ Hits         5707   5708    +1

Impacted Files	Coverage Δ
tests/test_additional_responses_router.py	100% <ø> (ø)	⬆️
fastapi/openapi/utils.py	100% <100%> (ø)	⬆️
fastapi/openapi/models.py	100% <100%> (ø)	⬆️

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