- Adds #each_chunk to ActionDispatch::Response. it's a method which
    will be called by ActionDispatch::Response#each.
  - Make Response#each a proper method instead of delegating to @stream
  - In Live, instead of overriding #each, override #each_chunk.
  - `#each` should just spit out @str_body if it's already set
  - Adds #test_set_header_after_read_body_during_action
    to prove this fixes #23964
  - Adds #test_each_isnt_called_if_str_body_is_written to
    ensure #each_chunk is not called when @str_body is available
  - Call `@response.sent!` in AC::TestCase's #perform so a test response
    acts a bit more like a real response. Makes test that call  `#assert_stream_closed`
    pass again.
  - Additionally assert `#committed?` in `#assert_stream_closed`
  - Make test that was calling @response.stream.each pass again by
    calling @response.each instead.