Rename parameter content_type to response_class

[tiangolo]

♻️ Rename parameter content_type to response_class.

[codecov]

Codecov Report

Merging #183 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #183   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         164    164           
  Lines        4035   4035           
=====================================
  Hits         4035   4035

Impacted Files	Coverage Δ
fastapi/applications.py	100% <ø> (ø)	⬆️
fastapi/openapi/utils.py	100% <100%> (ø)	⬆️
docs/src/custom_response/tutorial001.py	100% <100%> (ø)	⬆️
fastapi/routing.py	100% <100%> (ø)	⬆️
docs/src/custom_response/tutorial004.py	100% <100%> (ø)	⬆️

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