Do not issue warning when calling set_encoding if string is chilled #448

Job	Run time
ruby-versions / Generate Ruby versions	2s
ubuntu-24.04 2.7	42s
ubuntu-24.04 3.0	10s
ubuntu-24.04 3.1	11s
ubuntu-24.04 3.2	11s
ubuntu-24.04 3.3	11s
ubuntu-24.04 3.4	11s
ubuntu-24.04 debug	20s
ubuntu-24.04 head	9s
ubuntu-24.04 jruby-head	1m 3s
ubuntu-24.04 truffleruby-head	27s
ubuntu-22.04 2.7	37s
ubuntu-22.04 3.0	15s
ubuntu-22.04 3.1	13s
ubuntu-22.04 3.2	17s
ubuntu-22.04 3.3	15s
ubuntu-22.04 3.4	10s
ubuntu-22.04 debug	21s
ubuntu-22.04 head	10s
ubuntu-22.04 jruby-head	1m 1s
ubuntu-22.04 truffleruby-head	29s
	7m 25s

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