Rack env: prefer UTF-8 encoding when the Rack spec allows it #852

Job	Run time
Ruby ruby-head / Rack ~> 3.1	2m 27s
Ruby 3.3 / Rack ~> 3.1	2m 11s
Ruby 3.2 / Rack ~> 3.1	2m 23s
Ruby 3.1 / Rack ~> 3.1	2m 25s
Ruby 3.0 / Rack ~> 3.1	2m 20s
Ruby 2.7 / Rack ~> 3.1	2m 31s
Ruby 2.6 / Rack ~> 3.1	2m 39s
Ruby 3.3 / Rack ~> 3.1 --enable-frozen-string-literal	2m 15s
Ruby 3.3 / Rack ~> 3.0.0	2m 11s
Ruby 3.3 / Rack ~> 2.0	2m 41s
	24m 3s

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