Zlib.gunzip should not fail with utf-8 strings #86

Job	Run time
build (3.1 / ubuntu-latest)	16s
build (3.1 / macos-latest)	32s
build (3.1 / windows-latest)	1m 51s
build (3.0 / ubuntu-latest)	15s
build (3.0 / macos-latest)	40s
build (3.0 / windows-latest)	1m 35s
build (2.7 / ubuntu-latest)	34s
build (2.7 / macos-latest)	2m 14s
build (2.7 / windows-latest)	2m 14s
build (head / ubuntu-latest)	20s
build (head / macos-latest)	32s
build (truffleruby-head / ubuntu-latest)	5m 34s
build (truffleruby-head / macos-latest)	5m 56s
build (mswin / windows-latest)	2m 7s
build (mingw / windows-latest)	1m 28s
	26m 8s

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