Allow read-only methods to work against a frozen StringIO #428

Job	Run time
ruby-versions / Generate Ruby versions	2s
ubuntu-24.04 2.7	27s
ubuntu-24.04 3.0	9s
ubuntu-24.04 3.1	9s
ubuntu-24.04 3.2	10s
ubuntu-24.04 3.3	14s
ubuntu-24.04 3.4	9s
ubuntu-24.04 debug	15s
ubuntu-24.04 head	8s
ubuntu-24.04 jruby-head	52s
ubuntu-24.04 truffleruby-head	29s
ubuntu-22.04 2.7	23s
ubuntu-22.04 3.0	15s
ubuntu-22.04 3.1	10s
ubuntu-22.04 3.2	13s
ubuntu-22.04 3.3	13s
ubuntu-22.04 3.4	13s
ubuntu-22.04 debug	20s
ubuntu-22.04 head	16s
ubuntu-22.04 jruby-head	56s
ubuntu-22.04 truffleruby-head	31s
	6m 34s

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