Make `IO::Buffered#buffer_size=` idempotent #11928

Job	Run time
check_format	1m 46s
deploy_api_docs	0s
x86_64-gnu-test (1.7.3)	38m 53s
x86_64-gnu-test (1.8.2)	33m 7s
x86_64-gnu-test-preview_mt	5m 53s
x86_64-musl-test	38m 50s
x86_64-gnu-test (1.9.2)	34m 59s
x86_64-gnu-test (1.10.1)	36m 7s
x86_64-gnu-test (1.11.2)	32m 41s
x86_64-gnu-test (1.12.2)	33m 10s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 35s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	33m 46s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	35m 19s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	34m 46s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	33m 46s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	33m 52s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	35m 19s
	8h 14m 49s

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