Allow read-only methods to work against a frozen StringIO #427

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 2.7	24s
ubuntu-24.04 3.0	10s
ubuntu-24.04 3.1	14s
ubuntu-24.04 3.2	13s
ubuntu-24.04 3.3	12s
ubuntu-24.04 3.4	9s
ubuntu-24.04 debug	15s
ubuntu-24.04 head	13s
ubuntu-24.04 jruby-head	45s
ubuntu-24.04 truffleruby-head	30s
ubuntu-22.04 2.7	24s
ubuntu-22.04 3.0	10s
ubuntu-22.04 3.1	11s
ubuntu-22.04 3.2	11s
ubuntu-22.04 3.3	16s
ubuntu-22.04 3.4	9s
ubuntu-22.04 debug	16s
ubuntu-22.04 head	13s
ubuntu-22.04 jruby-head	44s
ubuntu-22.04 truffleruby-head	30s
	6m 13s

Provide feedback