Allow read-only methods to work against a frozen StringIO #369

Job	Run time
ruby-versions / Generate Ruby versions	2s
windows-latest 2.7	2m 28s
windows-latest 3.0	1m 30s
windows-latest 3.1	1m 25s
windows-latest 3.2	1m 43s
windows-latest 3.3	1m 45s
windows-latest 3.4	3m 4s
windows-latest jruby-head	2m 30s
windows-latest mingw	1m 27s
windows-latest mswin	2m 6s
	18m 0s

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