Avoid fast-path IO writes when IO has ext enc #843

Job	Run time
Ruby memcheck	2m 0s
ruby-versions / Generate Ruby versions	2s
ubuntu-latest 2.7	45s
ubuntu-latest 3.0	23s
ubuntu-latest 3.1	22s
ubuntu-latest 3.2	23s
ubuntu-latest 3.3	23s
ubuntu-latest 3.4	22s
ubuntu-latest head	1m 19s
ubuntu-latest jruby	1m 55s
ubuntu-latest jruby-head	2m 20s
macos-14 2.7	56s
macos-14 3.0	24s
macos-14 3.1	25s
macos-14 3.2	16s
macos-14 3.3	18s
macos-14 3.4	23s
macos-14 head	17s
macos-14 jruby	43s
macos-14 jruby-head	39s
windows-latest 2.7	3m 54s
windows-latest 3.0	2m 36s
windows-latest 3.1	2m 29s
windows-latest 3.2	2m 13s
windows-latest 3.3	2m 24s
windows-latest 3.4	4m 6s
windows-latest head	1m 47s
ubuntu-24.04-arm 3.4	35s
macos-13 3.4	29s
windows-latest mswin	1m 58s
ubuntu-latest jruby-9.4	1m 12s
macos-latest truffleruby-head	32s
ubuntu-latest truffleruby-head	1m 52s
	40m 42s

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