strio_read: preserve buffer encoding on partial reads #360

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-22.04 2.7	40s
ubuntu-22.04 3.0	13s
ubuntu-22.04 3.1	15s
ubuntu-22.04 3.2	12s
ubuntu-22.04 3.3	17s
ubuntu-22.04 debug	30s
ubuntu-22.04 head	15s
ubuntu-22.04 jruby-head	58s
ubuntu-22.04 truffleruby-head	31s
ubuntu-20.04 2.7	32s
ubuntu-20.04 3.0	14s
ubuntu-20.04 3.1	13s
ubuntu-20.04 3.2	15s
ubuntu-20.04 3.3	16s
ubuntu-20.04 debug	27s
ubuntu-20.04 head	15s
ubuntu-20.04 jruby-head	1m 0s
ubuntu-20.04 truffleruby-head	34s
	7m 41s

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